**6. Using animal models for mechanistic understanding of entry, invasion, and destruction of nervous system**

For a closer interrogation of the role of OE infection by CoV-2, gene expression patterns were studied in the mouse whole olfactory mucosa (WOM) and purified olfactory sensory neurons (OSNs). Single-cell sequencing of mouse WOM uncovered the expression of ACE2 and TMPRSS2 in the dorsally located SUS cells, basal globose cells as well as in a small fraction of the stem cells. Mouse OSNs, however, did not show the expression of the CoV-2 entry genes [6]. Within the OB as well, sequencing did not reveal any neuronal expression of these genes while the immunostaining displayed their expression in the pericytes of OB blood vessels. Postmortem magnetic resonance imaging (MRI) and histopathological examination confirmed microvascular injuries in the OB and brain stem of COVID-19-infected patients [120]. Such studies confirmed the olfactory trans-mucosal pathway of entry of CoV-2 virus into the body [3].

Using the hamster infection model of CoV-2 invasion, nucleoprotein expression was found out in Tuj1-positive infected OSNs and OMP staining also confirmed infection in mature OSNs [121]. Along with local inflammation of OE, SARS-CoV-2 infected the Tuj-1 positive immature OSNs, which appeared to be phagocytosed by the Iba1 and CoV-2-positive immune cells. In fact, global chromatin rearrangements occurred at day 3 post infection, which persisted even after the virus was cleared (10 days post infection) [122]. Whether the interferon response in the OSNs can bring about such chromatin changes in CoV-2 scenario remains an open question. It is also yet to be deciphered if other actively dividing cells are also prone to such disruptions and if not, then what makes certain cell types more unique and susceptible to viralinduced genomic modifications [123].

Nasal irrigation of virus in Golden Syrian Hamsters once again revealed the neuronal invasion. The OMP positive neuronal cilia were vanished in the virus-induced damaged epithelium of the animals. Both neuronal and non-neuronal cells were found out to be positive for cleaved caspase-3 after 4 days of infection, which was suggestive of cell death. Additionally, CoV-2 nucleoprotein was found at the junction of olfactory nerve and the OB and also in previously uncharacterized cells of the glomerular layer of the OB, suggestive of, invasion of the bulb by the virus [124]. Finally, to grasp the molecular underpinnings of the olfactory impairment upon viral infection, transcriptional changes in OE cells of the infected hamsters were studied. A significant reduction in the OR genes, i.e., the genes responsible for olfaction, was observed. At genomic level, they found out a drastic change in the long-range interactions of the OR genes with enhancer/activators 1 day post infection in a non-cell-autonomous fashion.
