Introduction

Coronavirus disease 2019 (COVID-19) is the term used by the World Health Organization (WHO) to refer to the unfolding pandemic caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). It was first described in December 2019 in Wuhan, Hubei Province, China, as pneumonia of unknown cause [1]. Since then, its rapid spread worldwide has become a global health, economic, and humanitarian crisis. The WHO declared COVID-19 a global emergency on January 30, 2020. Subsequently, on March 11, 2020, it was designated as a global pandemic. As of May 2021, it caused over 159 million global cases and ~ 3.3 million deaths in 219 countries and territories around the world (Fig. 1). Although, most people infected with SARS-CoV-2 are either asymptomatic or exhibit mild-to-moderate symptoms, but some of the cases do develop serious and even fatal medical complications. The main clinical neurological manifestations in SARS-CoV-2 infections are fever, dry or productive cough, fatigue, dyspnea, myalgias, headache, sore throat, diarrhea, chills, nausea/vomiting, and rhinorrhea (Fig. 1). However, loss of smell and taste with COVID-19 infection was the most prominent neurological symptom displayed in the patients, and the research has suggested that the inflammation in the olfactory neurons may be linked to this condition. In this review, we have focused on the plausible pharmacological mechanisms associated with the COVID-19 disease progression in perspective of its neurological complications. The incubation period for COVID-19 ranges from 1 to 14 days; however, few cases (less than 1%) have also been reported to develop symptoms after 14 days [2]. Around 75% of cases develop symptoms between 2 and 7 days after exposure with a median incubation period of 4–5 days [3]. Most COVID-19 cases (81%) that experience mild-to-moderate symptoms recover without hospitalization [4] (Fig. 1). Approximately 19% of patients develop severe symptoms and require hospitalization, typically on day 6–8. Few cases (5%) develop critical conditions like respiratory failure, shock, and even multi-organ failure. Generally, the aged population or people with existing chronic medical conditions are at high-risk of developing life-threatening complications with SARS-CoV-2 infection. It is very clear about the incidence rate and patient symptoms of COVID-19 infection, but the uncertainty in the development of this disease challenges us to write this article correlating with existing literature and signifying the importance of fewer molecular targets.

Fig. 1
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Schematic of the incidence of neurological symptoms and simplified coronavirus replication cycle showing the possible therapeutic targets with potential repurposed drugs. ACE2, angiotensin-converting enzyme carboxypeptidase 2

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Structure and Viral Entry into Host

SARS-CoV-2 is a novel coronavirus and is the seventh strain of coronavirus known to infect humans. It is a positive-sense, single-stranded RNA virus and belongs to the genus Betacoronavirus, which also includes SARS-CoV and MERS-CoV [5]. The club-shaped spike projections, originating from the surface of the envelope that contains nucleocapsid, are the most prominent feature of these viruses. The main structural proteins of the coronavirus particles are spike (S), membrane (M), envelope (E), and nucleocapsid (N) proteins [5]. The glycoprotein spikes (S protein), present on the surface of the SARS-CoV-2, aid in the binding of the virus to its cellular receptors, the angiotensin-converting enzyme carboxypeptidase 2 (ACE2) (Fig. 1). Subsequently, the conformational changes in the S protein facilitate the internalization, and the ACE2-virus complex is then translocated to endosomes. The fusion of the viral envelope to the cell membrane occurs within acidified endosomes. This ultimately results in the release of viral genomic RNA to the cytoplasm, which is then translated into polypeptides. These polypeptides undergo proteolysis to make nonstructural proteins, some of which form a replication-transcription complex. This complex uses the genomic RNA as a template to replicate newer strands of genomic RNA and to produce subgenomic RNAs by discontinuous transcription on the same template RNA strand. The subgenomic RNAs are translated into structural proteins. The genomic RNA combines with the N protein to form nucleocapsid in the cytoplasm whereas spike (S), envelope (E), and membrane (M) proteins enter the endoplasmic reticulum. Finally, in the endoplasmic reticulum-Golgi intermediate compartment (ERGIC), they assemble with the nucleocapsid to make complete virus particles. Mature virions are released to the extracellular region through exocytosis (Fig. 1). These matured virions further contribute to the cellular damage of the host and thereby exhibits pathogenecity of the infection.

SARS‐CoV‐2: Neurovirulence and Central Nervous System Entry Routes

It was articulated that COVID-19 infection involves a number of neurological symptoms which contribute to morbidity and mortality in infected patients. According to the published reports, the headache, dizziness, impaired consciousness, acute cerebrovascular disease, epilepsy, ataxia, acute disseminated encephalomyelitis (ADEM), viral encephalitis, hyposmia/anosmia, hypogeusia/ageusia, muscle pain, and Guillain–Barre syndrome (GBS) are considered neurological manifestations of COVID-19 infection. The severity of these symptoms is varied depending on overall health status of COVID-19 patients.

Despite the high prevalence of neurological symptoms and encephalopathy-associated morbidity in COVID-19 patients, the mechanisms underlying these neurological symptoms/manifestations are unclear. SARCoV-2 seems to affect the CNS directly by invading the brain and also indirectly as a consequence of its systemic effects [6] (Fig. 2). The host immune response to the virus itself may affect the CNS by precipitating virus-induced neuropsychiatric sequelae. However, growing evidence is suggesting the neurovirulence of SARS‐CoV‐2 and its role in neuropsychiatric complications [7]. The identification of SARS‐CoV‐2 RNA in the cerebrospinal fluid of COVID-19-associated encephalitis patients provided the most conclusive evidence to document the neurovirulence of SARS‐CoV‐2 [8]. Recently, transmission electron microscopy of the brain tissue of a 74-year-old male with SARS-COV-2 showed evidence indicating the neuroinvasive nature of the virus and the likely routes of transmission to the CNS [9].

Fig. 2
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SARCoV-2 mediated neurological manifestations. ACE2, angiotensin-converting enzyme carboxypeptidase 2

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The key questions for SARS-CoV-2 infection concern the routes of entry into the CNS. One of the probable routes of entry for SARS-CoV-2 into brain tissues could be via the olfactory bulb, which is the only part of the CNS not protected by dura. The presence of anosmia and hyposmia in patients with COVID-19 also supports the idea of SARS-CoV-2 dissemination and spread from the olfactory nerves to the brain [10, 11]. Baig et al. have also suggested that an upper nasal transcribable route enables the SARS‐CoV‐2 to reach the brain [10]. Several studies using experimental animal models have also shown that viruses like human coronavirus OC43 (HCoV-OC43), SARS-CoV-2 and Middle East respiratory syndrome coronavirus (MERS‐COV) could enter the brain following intranasal injection possibly via olfactory nerves with subsequent dissemination to distant area by transneuronal retrograde transport [12,13,14,15]. Similarly, SARS-CoV-2 may also spread to the brain by transneuronal anterograde and retrograde transport via afferent nerve endings of the vagus nerve from lungs [16] and enteric nerve and sympathetic afferents from the gastrointestinal tract [17,18,19]. Moreover, exosomal cellular transport may also be one of the presumed pathways of systemic dissemination of viruses and their subsequent CNS entry [20]. Alternative entry routes include passage across the blood–brain barrier, following viremia, or through infected leukocytes [21,22,23].

The presence of the virus in general circulation enables its entry into cerebral circulation, where slow blood movement in microvessels aids in the binding of viral spike protein with ACE2 receptors present on endothelial cells [10]. This subsequently leads to viral budding from capillary endothelium; resultant damage to the endothelial lining favors viral entry into the milieu of the brain. The angiotensin-converting-enzyme 2 receptor, to which SARS-CoV-2 binds for entry into cells [24], is found in the brain vascular endothelium and smooth muscle [25]. SARS-CoV-2 has been shown to replicate in in vitro neuronal cells [26].

SARS-CoV-2 entry in the brain either via hematogenous route or via neuronal route may lead to cellular dysfunction in the brain cells [27]. ACE2, the functional receptor for SARS-CoV-2, expression in the brain is not only limited to the vasculature but it is also expressed in the neuronal and glial cells [28, 29]. Once in the CNS, it can infect neurons and microglia activating the cascade of neuroinflammation and neurodegeneration through the release of tissue tumor necrosis factor-alpha (TNFα), cytokines, reactive oxygen species (ROS), and other inflammatory mediators. Activated glial cells are well-known to contribute to neuroinflammation by producing pro-inflammatory cytokines like interleukin-6 (IL-6), IL-2, IL-5, and TNFα [30]. Similarly, neuronal production of IL-6 in vivo following viral infection has been reported [31]. SARS-CoV-2 also activates CD4+ cells in the CNS which in turn induces the macrophage to secrete IL-6. IL-6 is a predominant component of the cytokine storm, which is the major cause of COVID-19 associated complications [32]. It is well-established that excessive production of cytokines adversely affects the cellular metabolism in the brain and other organs/tissues. This is further supported by the fact that treatment with tocilizumab (an IL-6 receptor blocker) resulted in the improvement of critically ill patients with COVID-19 [33]. Aforesaid facts suggest that cytokine storm syndrome is one of the main mechanisms associated with SARS-CoV-2-induced brain damage.

SARS-CoV-2 mediated endothelial cell damage of cerebral vasculature and resultant cerebrovascular dysfunction could be another major reason for COVID-19 associated neurological manifestation [6, 27]. Although cerebral vasculature has not yet been investigated; however, endothelial cell infection, inflammation and apoptosis have been seen in various organs including lungs, heart, kidney and bowel at autopsy in patients with COVID-19 [34,35,36].

SARS-CoV-2-induced endothelial cell injury can strongly trigger the coagulation cascade via activation of various pro-coagulation pathways including activation of tissue factor [37, 38]. COVID-19-associated endothelial dysfunction can potentially lead to microvascular and macrovascular complications in the brain [39]. The endothelial damage of cerebral vasculature and subsequent bleeding can have fatal consequences in patients with COVID-19 even without any visible neuronal damage [40]. The persistent inflammatory status in critical patients with COVID-19 patients could also trigger the coagulation cascade. Certain pro-inflammatory cytokines, including IL-6, could activate the coagulation system and suppress the fibrinolytic system. Monocyte activation is postulated to constitute part of the secondary hemophagocytic lymphohistiocytosis described in severe COVID-19 [41]. Thrombocytopenia with elevated d-dimer and C-reactive protein in severe COVID-19 and stroke is consistent with a virus-associated microangiopathic process [42]. Higher production of anti-cardiolipin and anti-β2GP1 antibodies has been also reported in patients with COVID-19 [43]. A recent study has suggested that high morbidity and mortality in COVID-19 patients may be associated with high incidence of acute cerebrovascular disease. This study also suggested that thrombotic microangiopathy in patients with COVID-19 is caused by endotheliopathy with a hemorrhagic predisposition [44]. All these evidences suggest that SARS-CoV-2 mediated endothelial dysfunction of cerebral vasculature and may also contribute to COVID-19-associated neurological manifestations.

Role of COVID-19 in Redox Imbalance and Mitochondrial Dysfunction

Various published literature strongly supports the fact that mitochondrial dysfunction is very apparent in viral infections and may be contributing to immune system dysfunction in the host [45]. However, it is also evident that ACE2 which cleaves Angiotensin II into Angiotensin I to VII plays a vital role in maintaining mitochondrial homeostasis [46] (Fig. 3). An ACE2 knockout mouse has displayed impaired mitochondrial respiration and ATP production [47]. Moreover, NADPH oxidase (Nox4) has been identified as a source of O2− in neuron mitochondria that contributes to ANG II intraneuronal signaling [48]. However, ACE2 importance in COVID-19 entrance into the host immune cells is well-established verity in 2020 [49,50,51]. It is very logical to assume downregulation of ACE2 by COVID-19 may impact on mitochondrial function of immune cells and thereby may reduce the host immune function. Nevertheless, ACE2 has been shown to be under-expressed in experimental models of chronic diseases like diabetes [52], hypertension [53], cardiac myopathy [54], renal dysfunction [55], and neurodegenerative diseases [56]. Our research findings along with others also articulated the significance of mitochondrial dysfunction in the progression of diabetes and its complications: hypertension, Alzheimer’s, Parkinsonism, Huntington’s disease, cardiac myopathy, etc. [57,58,59]. These findings may force us to think that a higher mortality rate is seen in patients with serious COVID-19 infection and suffering from these comorbidities may be associated with mitochondrial dysfunction (Fig. 3). Contrastingly, mitochondrial dysfunction is also found to be independent of the expression of ACE2 in platelets of severely infected COVID-19 patients [60]. This observation states that mitochondrial dysfunction is very significant in COVID-19 disease progression but ACE2 may not be the only for it reason in all infected host cells. Recently, Hoffman et al. demonstrated COVID-19 membrane–associated glycoprotein; Transmembrane serine protease 2 (TMPRSS2) binds with host ACE2, thus helping the virus entry into the host cells [61]. A growing body of evidence suggests that TMPRSS2 also participates in maintaining mitochondrial homeostasis via stimulation of estrogen-related receptor alpha (ERRα)—peroxisome proliferator-activated receptor gamma coactivator 1-alpha (PGC1α)—mitochondrial transcription factor (TFAM) axis [62, 63]. PGC1α is considered the master regulator of mitochondrial function and mitochondrial biogenesis [64]. PGC1α knockout mice in different experimental conditions displayed reduced mitochondrial complex activities, ATP production, and dysregulated mitochondrial unfolded response as depicted in Fig. 3 [65, 66]. Based on these observations, there can be a possibility of curtailing COVID-19 infection via boosting PGC1α levels. During COVID-19 infection, it is reported that progressive loss of endothelial barrier in the capillaries supplying the lungs leverages the accumulation of fluid and macromolecules in the interstitium and alveolar spaces which ultimately leads to pulmonary edema formation and in this manner causing severe breathlessness. Adenosine monophosphate–activated protein kinase (AMPK) is considered a metabolic manipulator and critical regulator of endothelial cell function and maintenance of endothelial barrier integrity [67]. A knockout mouse with AMPK has shown significant lung edema, congestion, and inflammation [68]. AMPK activators like AICAR or metformin treatment improved lung function by improving endothelial cells functioning in the blood capillaries supplying blood to the lungs [69, 70]. There are enough literary evidence that has suggested the central role of AMPK in regulating mitochondrial homeostasis [71]. AMPK gets activated in response to the loss of mitochondrial function as an adaptive mechanism; however, during a chronic disease status, AMPK gets compromised where the cellular fate changes with series of pathological events from that of mitochondrial dysfunction to apoptosis [72]. AMPK by activating its downstream signaling proteins maintains mitochondrial function, mitochondrial biogenesis, and mitophagy as shown in Fig. 3. COVID-19 infection downregulates nuclear-encoded mitochondrial genes related to cellular respiration and Complex 1 assembly where they are linked to upstream AMPK activity [73]. Therefore, AMPK activation may be helpful in reducing COVID-19 infection associated with inflammation and mitochondrial dysfunction, and in-depth experimental studies are warranted to understand its role (Fig. 3). Another characteristic feature of COVID-19 infection is the cytokine storm, which is a hyper-inflammatory state of the disease. In patients infected with COVID-19, blood analysis has shown increased TNF-α and inflammatory interleukins which include IL-1β, IL-6, and IL-10 [74]. Several lines of evidence suggested TNFα role in enhancing the mitochondrial ROS generation triggering through calcium-dependent pathways [75]. Additionally, it has also been reported that mitochondrial ROS is one important factor triggering pro-inflammatory cytokines like IL-1β production. IL-1β once induced can lead to inflammasome formation (NLRP3) and may further amplify the persistent inflammation in patients with COVID-19 infection [76]. We also know about the possible involvement of interferon-1 (IFN-1) against viral infections as a part of inbuilt immune defense mechanism [77]. When immune cells are exposed to RNA viruses, an intracellular antiviral response is triggered by activation of RIG-I-like receptors. Thereby, RIG-1-like receptors trigger downstream signaling complex on the mitochondrial outer membrane including the MAVS-TOM70 axis. This can boost the immune response against host viruses via upregulating IFN-β production [78]. Indeed, alternative open reading frame (Orf9) that exists within the nucleocaspid gene of SARS-CoV viruses can significantly inhibit the production of IFN-1 from host mitochondria [79]. Moreover, recent research findings on SARS-CoV-2-infected patients showed increased serum IFN-1 levels. Nevertheless, SARS-CoV-2 Orf9b suppressed type I interferon responses by targeting TOM70 [80]. Altered mitochondrial membrane potential during the viral infections leaks cytochrome c from the mitochondrial matrix into the cytosol [81]. Cytochrome c activates the caspase cascade pathway that leads to programmed cell death or apoptosis [82]. This would be another reason for multiple organ failure–related or breathlessness-related deaths in patients with COVID-19.

Fig. 3
figure 3

Schematic picture showing the plausible mechanism of COVID-19 in mitigating mitochondrial dysfunction. COVID-19 infection stimulates NF-κB pathway and inhibits Nrf2 pathway thereby leads to the redox imbalance and enhances the production of cytokines (TNF-α, IL-1β, IL-6, and IL-10). In turn, TNF-α by acting on its surface receptor enhances the generation of mitochondrial ROS. On another hand, AT1 receptor activation by Ag II formed by ACE involves in regulating AMPK pathway and its downstream mediators controlling mitochondrial function and mitochondrial biogenesis. During the progression of COVID-19 infection, viruses utilize ACE to enter into the host cells and make it unavailable for normal cellular functions, where directly impacts the Ag II homeostasis in controlling cellular functions. This may dysregulate mitochondrial function, perturb mitochondrial biogenesis, and may cause mitochondrial proteotoxicity during COVID-19 infection. Ag II, Angiotensin II; ARE, antioxidant-responsive element; AMPK, adenosine monophosphate–activated protein kinase; Atg, anti-thymocyte globulin; FIP200, FAK family kinase-interacting protein of 200 kDa; GpX, glutathione peroxidase; HIF-1α, hypoxia-inducible factor 1-alpha; HO1, heme oxygenase 1; mTOR, mechanistic target of rapamycin; NQO1, NAD(P)H dehydrogenase [quinone] 1; SIRT1, silent mating–type information regulation 2 homolog 1; NRF, nuclear respiratory factor; Nrf2, nuclear factor erythroid 2 (NFE2)–related factor 2; NF-κB, nuclear factor kappa-light-chain-enhancer of activated B cells; PGC-1α, peroxisome proliferator-activated receptor gamma coactivator 1-alpha; Rheb, Ras homolog enriched in the brain; SOD2, superoxide dismutase 2; TFAM, mitochondrial transcription factor; TSC, tuberous sclerosis proteins; Ulk1, unc-51-like autophagy activating kinase; VPS34, vacuolar protein sorting 34

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Mitochondrial ROS directly attack mitochondrial proteins and thus rendering them non-functional. Normally, cells regulate ROS production by enhancing the phase II detoxifying enzymes or also considered antioxidant enzymes; superoxide dismutase (SOD), catalase, NAD(P)H dehydrogenase [quinone] 1 (NQO-1), glutathione peroxidase (GpX), glutathione-S-transferase (GST), and heme oxygenase (HO1) which performs the function of neutralizing the ROS [83]. In this regard, it is very important to discuss nuclear erythroid–related factor 2 (Nrf2) cap ‘n’ collar transcription factor that directly binds to antioxidant responsive elements (ARE) to regulate the gene expression of these antioxidant enzymes [84]. There are several drugs that directly activate Nrf2 under clinical trials for the treatment of more than one indication like sulforaphane and bardoxolone methyl [85, 86]. In previous studies, Nrf2 was under-expressed in human coronavirus HCoV-229E which is associated with the common cold and pulmonary edema [87]. Interestingly, Nrf2 suppression was also found in the lung biopsies of patients with COVID-19 [88]. In fact, ACE2 expression is also regulated by Nrf2 pathway, and few studies demonstrated that Nrf2 under-expression increases the availability of ACE2 and that may increase the risk of viral infection [89]. Mice infected with SARS-CoV have been shown increased NF-κB pathway and produced significant pulmonary inflammation [90]. Many experimental studies advocate a cross-talk between Nrf2 and NF-κB; activation of Nrf2 suppresses the NF-κB activity and subsides over the inflammatory buildup [91, 92]. Additionally, upregulation of HO-1 has been shown to have protective action against many viruses including HIV, hepatitis B and C virus, Enterovirus 71 (E71), influenza virus, respiratory syncytial virus [93], dengue virus (DENV), and Ebola virus (EBOV) [94]. Nrf2 also induces mitochondrial specific antioxidant defense systems like superoxide dismutase-2 (SOD2) which plays a vital role in regulating mitochondrial superoxides [95]. Of note, collective evidence point out Nrf2 would be another potential target for the treatment of COVID-19 infection. Few studies demonstrated that pulmonary complication during COVID-19 infection imparts hypoxia that may cause upregulation of hypoxia-inducible factor-alpha (HIF-1α). HIF-1α downregulates ACE2 levels which may be an adaptive mechanism for the treatment of COVID-19 infection. HIF-1α also have been implicated in regulating heat shock proteins (HSPs) which contribute to repairing misfolded proteins in the mitochondria and impairs mitochondrial dysfunction [96, 97]. Therefore, HIF-1α also may be an important target in manipulating SARS-CoV-2 infection (Fig. 3).

Drugs Under Investigation for COVID-19 Infection Targeting Oxidative Stress, Inflammation, and Mitochondrial Dysfunction

Oxidative stress and inflammation are well-established hallmarks for COVID-19 infection in human beings. Current research in exploring the suitable drug candidates for the treatment of COVID-19 infection is limited with poor availability of experimental models because the SARS-CoV-2 cannot infect wild-type laboratory mice owing to inefficient interactions between the viral spike protein and the mouse orthologue of the human receptor, ACE2. Now, the researchers established a genetic mice model that allows the interaction between SARS-CoV-2 spike proteins and mouse ACE2 and designed mouse-adapted SARS-CoV-2, a recombinant virus that can use mouse ACE2 for entry into cells. This model supported the clinical usage of pegylated IFN-λ1a as a treatment for human COVID-19 infection and under usage. Moreover, several animal models for SARS-CoV-2 have been reported, with varying degrees of viral replication and clinical disease, including ACE2 transgenic mice and virally transduced ACE2 mice, which express human ACE2, ferrets, hamsters, and non-human primates. mRNA vaccines are new type of vaccines which guides the immune cells to synthesize the harmless spike proteins that boosts up the immune system to synthesize antibodies against them and also oppose the actions of actual COVID-19 spike proteins. FDA-approved Moderna and Pfizer/BioNTech’s COVID-19 mRNA vaccine were approved on emergency basis in the USA. The up-to-date research suggests that a prospective drug will be which can affect the viral entrance into the host cells or the drugs previously reported for their antioxidant potentials or drugs used for treatment of inflammatory diseases. Drug repurposing for the treatment/management of COVID-19 infection can lessen the time required for availability of therapy for COVID-19. The summary of the current ongoing trials is summarized in Table 1. These agents are known to target oxidative stress, inflammation, and mitochondrial dysfunction.

Table 1 The potential repurposing drugs under investigation for the treatment of COVID-19 infection. AMPK, Adenosine monophosphate–activated protein kinase; AT1, Angiotensin 1; mTOR, mechanistic target of rapamycin; IL, interleukin; Nrf2, nuclear factor erythroid 2 (NFE2)–related factor 2; SIRT1, silent mating–type information regulation 2 homolog 1; TNF, tumor necrosis factor; TMPRSS2, active transmembrane serine protease 2
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Conclusion

This paper reviewed the neurological symptoms associated with COVID-19 infection and presented various mechanisms of SARS-CoV-2 infection related to redox imbalance and mitochondrial dysfunction. Maintaining the redox homeostasis or combating the mitochondrial dysfunction may be as critical as targeting the virus. Treatment strategies that can check the viral entry into the host cell or inhibit the viral infection can be complemented by using strategies like boosting antioxidant defense, improving mitochondrial health and function, and combating the inflammatory surge not only can prevent the viral infection but also can help in reducing the aftermath of viral infection. Further molecular studies deciphering how the agents targeted at redox homeostasis, AMPK modulation, or improving mitochondrial functioning can help in checking the viral entry to host cell are associated tissue/organ damage in COVID-19 infection are warranted.