**1. Introduction**

The Coronavirus disease 2019 (COVID-19) pandemic poses an unparalleled challenge to the public health in dealing with long-term adverse effects of the infection. Several neurological complications have been reported to be associated with COVID-19. At the beginning of pandemic, in one of the early correspondences on autopsy studies published in the New England Journal of Medicine reported the presence of SARS-CoV-2 in multiple organs including the brain [1]. Recently published brain imaging data from subjects who were scanned before and after infection show structural abnormalities in the central nervous system (CNS). Significant changes were found in the brain areas that are functionally connected to the primary olfactory cortex, orbitofrontal cortex, and olfactory tubercle. This suggests possible long-term cognitive impairments due to COVID-19 infection in the central nervous system (CNS) that may happen through olfactory mucosa [2]. These findings support early reports on the presence of SARS-CoV-2 RNA and protein in the nasopharynx [3]. However, the postmortem studies of olfactory and respiratory mucosa confirmed

sustentacular and ciliated cells as the targets for SARS-CoV-2 infection. There were no evidences found in this study for the presence of viral particles in the olfactory sensory neurons (OSNs) or olfactory bulb (OB), questioning the neurotropism shown by the virus [4]. These contrasting results prompt us to carry out a narrative literature review on the reported causalities and realities on the neurotropic characteristics of SARS-CoV-2.

One of the virus entry routes, i.e., binding of viral spike (S) protein to the human angiotensin-converting enzyme 2 (hACE2) receptor and the S protein priming by host cell transmembrane protease, serine 2 (TMPRSS2) was uncovered at the beginning of pandemic [5]. These cellular factors are present in the non-neuronal cells of human olfactory epithelium, cortical neurons, Purkinje neurons, cerebellar and cortical astrocytes, etc. [6, 7]. Another receptor type that can mediate the infection, Neuropilin-1 (NRP1), is abundantly found in the neurons, olfactory epithelial cells, and endothelial cells [8, 9]. Other potential route can be through the ACE2 receptors present on the endothelial cells, thereby using the vascular system to attack the blood-brain barrier and to get access to the CNS [10]. Thus, despite the entry route to CNS being a debated topic, these evidences can be used to explain the pathophysiology of neurological impairments and long-term cognitive dysfunctions caused by COVID-19 infection. In this chapter, we are summarizing the evidences for the debated topic of SARS-CoV-2 neurotropism, the importance of quantifying olfactory and cognitive fitness in the context of Neuro-COVID and the studies in model systems that suggest neurotropism. To this end, we have carried out the literature review using a combination of keywords such as "SARS CoV2 entry routes to brain" and "Olfactory and cognitive impairments due to COVID-19" and "animal models of CoV-2." We have mostly used Google Scholar and PubMed to search for the articles. As we are aiming to provide a narrative overview on the debated topic of Neurotropic SARS-CoV-2, we are summarizing only the selected and relevant findings on this topic.

### **2. Entry routes of SARS-CoV-2**

To investigate the pathophysiology associated with SARS-CoV-2 infection, one of the critical steps is to mechanistically discern the routes of its entry into the host. Unprecedented research is underway, since the beginning of the COVID-19-induced pandemic to tease out the different entry points of the novel SARS-CoV-2 in the human body. It has been confirmed that CoV-2 virus presents the spike glycoprotein to the cell membrane for binding to the human angiotensin-converting enzyme 2 (hACE2) receptor [11, 12]. It is famously referred to as the SARS-CoV functional receptor [13]. One of the imperative functions of hACE2 protein is maintaining the neural homeostasis by regulating the renin-angiotensin signaling (RAS) system [14]. A seamless entry into the cell is warranted by the cleavage of S2' site of the virus by the TMPRSS2 after engaging with hACE2 at the membrane [5, 15, 16]. In the endosomal compartments of the cell, the cleavage is mediated by Cathepsin L protease, which initiates formation of the fusion pores [17, 18]. Inside Golgi apparatus, Furin protease cleaves the virus into S1 and S2 compartments [19]. After successful entry and proteolytic cleavage, viral machinery is assembled and activated to spread the infection [16].

Importantly, the agents that allow SARS-CoV-2 entry, specifically, human-ACE2 (hACE2) are present across different bodily tissues including the brain [14]. Such a widespread expression in the body would allow for conjecturing several routes by which virus can enter and invade. Indeed, the repertoire of symptoms associated with

#### *Neurotropic SARS-CoV-2: Causalities and Realities DOI: http://dx.doi.org/10.5772/intechopen.108573*

COVID-19 is a testimony to the tropism of virus in different cell types and tissues. Studies involving bulk and single-cell RNA sequencing revealed ACE2-TMPRSS2 expression in the different cell-types such as the sustentacular (SUS) cells, respiratory ciliated and secretory cells as well as the horizontal basal cells of the respiratory and olfactory epithelium (RE and OE) of human nasal mucosa [20, 21]. Other peripheral routes include that of the eye and oral tissues [22, 23]. Virus specimen was found to be present in the conjunctivital and tear swab of patients [24, 25]. Indeed, the viral entry machinery components, ACE2 and TMPRSS2, are present in conjunctivital epithelium and the epithelial and endothelial parts of the cornea [26]. Oral cavity also allows viral entry due to the enrichment of the entry proteins in the epithelial cells of the salivary glands and mucosae found in the single-cell RNA sequencing data of human samples [27]. Entry via oral route suggests correlation of salivary viral titer with the taste loss observed in COVID-19 patients [27, 28].

CoV-2 virus can potentially breach the blood-brain barrier (BBB) as a result of the barrier instability caused due to the increased number of inflammatory cytokines upon infection [10, 29]. Viral invasion of the brain areas by gaining entry from the circumventricular organs (CVOs) and brainstem structures could also serve as plausible routes in the patients who suffer from massive cytokine storm or those having compromised health prior to the infection [30]. One of the cytokines, tumor necrosis factor-α (TNF-α) can enter the BBB or in CVOs (structures lining the ventricles with accessible vasculature), which can activate downstream microglia and astrocytes [31]. The activated cells, in turn, can cause damage to the neurons via excitotoxicity and thereby impair the signaling processes of the brain [30]. Fecal-oral routes are yet another proposed route of viral dissemination in the body [32]. It is, however, not confirmed that this transmission route is responsible for gastrointestinal symptoms associated with COVID-19. It has been hypothesized that movement via vagal and spinal axonal fibers can allow viral invasion of the GI tract. Occurrence of syncope in patients with normal electrocardiogram assessment hints toward changes in the neural control of blood pressure changes [33, 34]. In one study, patients with syncope indeed had a significantly lower increase in the compensatory heart rate compared with those non-syncope ones, which suggested plausible impairment in the baro-reflexive control. Such an acute hypocapnic hypoxemia could have occurred due to CoV-2-mediated ACE2 internalization in specific midbrain and medullary nuclei, which can lead to impairments in baroreflex and chemoreceptor responses [33]. Malfunctioning of brain-lung axis can be indicated as severe lung and chest CT abnormalities and defects observed in neuroimaging analysis. The sensory neurons lining the airways can sense the virus induced inflammatory responses in the lungs and provide feedback to the brain [35, 36]. The carotid body sinus nerves innervating this organ can profoundly play a role in the retrograde transport of the virus to the brain. Carotid body invasion by virus due to local expression of ACE2 can cause impaired peripheral arterial chemoreception leading to hypoxic and hypercapnic conditions with changes in pH [37]. Separately, the mucosal immune system, which comprises the lymphoid tissues of the gut and the lungs, provides the clue for dissociating the components of gut-lung axis in mediating dysfunctions associated with the COVID-19 infection. Translocation of the active immune cells from gut to lungs can exacerbate inflammation, even causing lung injuries and respiratory distress. Within the gut, CoV-2 can downregulate ACE2 expression causing microbial dysbiosis and further affecting lungs via the gut-lung axis. Finally, neural control of the cardiovascular processes under COVID-19 infectious condition is yet another axis via which the virus can act and affect these organ systems [38]. Cardiac arrhythmias in COVID-19

#### **Figure 1.**

*Neuroinvasion by SARS-CoV-2 and subsequent dysfunctions. (A) Infection of a healthy subject by SARS-CoV-2 leads the entry of virus into different organ systems. (B–G) Routes and cell-types through which the virus can enter and invade the nervous system, i.e., via, olfactory route, via blood-brain barrier, eye, choroid plexus, blood-cerebrospinal fluid (CSF) barrier, via lungs to the brain and through the gastric enterocytes to the central nervous system [7, 40–42]. (H) Altered homeostasis can occur as a result of neuroinvasion by CoV-2 virus leading to detrimental effects at multiple cell types of the nervous system, i.e., neuron (yellow), astrocyte (blue) and microglia (green) [43]. (I) System-wide dysfunctions ranging from cellular to olfactory to CNS and PNS pathologies have been reported in COVID-19 patients. (J) An infected COVID-19 patient with different bodily systems affected due to the viral tropism.*

patients are mostly occurring due to direct myocardial damage by CoV-2 infection or via the systemic inflammatory responses [39]. Arrythmias can also indirectly be caused by dysfunctional neural control of the heart rate. There are feedback mechanisms to the brain for maintenance of the cardiac rhythm and for dampening the production of cytokines and other inflammatory mediators in case of infection [38]. It could be that the severe CoV-2 infection can alter the neural feedback mechanisms of cardiovascular control. Apart from the olfactory route, these three axes i.e., the lung-brain, gut-lung, and heart-brain may also serve as the routes of transmission and invasion by the virus leading to multiple organs dysfunctions and manifestation of a variety of symptoms and conditions (**Figure 1**).
