Role of SARS-CoV-2 Spike-Protein-Induced Activation of Microglia and Mast Cells in the Pathogenesis of Neuro-COVID

Abstract

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) causes coronavirus disease 2019 (COVID-19). About 45% of COVID-19 patients experience several symptoms a few months after the initial infection and develop post-acute sequelae of SARS-CoV-2 (PASC), referred to as “Long-COVID,” characterized by persistent physical and mental fatigue. However, the exact pathogenetic mechanisms affecting the brain are still not well-understood. There is increasing evidence of neurovascular inflammation in the brain. However, the precise role of the neuroinflammatory response that contributes to the disease severity of COVID-19 and long COVID pathogenesis is not clearly understood. Here, we review the reports that the SARS-CoV-2 spike protein can cause blood–brain barrier (BBB) dysfunction and damage neurons either directly, or via activation of brain mast cells and microglia and the release of various neuroinflammatory molecules. Moreover, we provide recent evidence that the novel flavanol eriodictyol is particularly suited for development as an effective treatment alone or together with oleuropein and sulforaphane (ViralProtek^®), all of which have potent anti-viral and anti-inflammatory actions.

1. Introduction

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) causes coronavirus disease 2019 (COVID-19), leading to complex immune responses [1,2] that involve the release of several inflammatory cytokines/chemokines [1,3,4,5,6,7,8], especially interleukin-1beta (IL-1β) [9] and IL-6, often referred to as “cytokine exacerbated production” [3,10,11]. Almost 50 percent of patients infected with SARS-CoV-2 experience post-acute sequelae of SARS-CoV-2 (PASC) [12,13,14] shortly after the initial infection [15], known as “Long-COVID syndrome” [16,17,18]. A recent report indicated that about 45% of COVID-19 survivors showed persistent symptoms at about 4 months in post COVID-19 population, with fatigue being most frequently experienced in hospitalized and non-hospitalized cohorts [14]. Long COVID is characterized by persistent fatigue that is not dependent on the initial severity of the disease [19] and presents with persistent symptomatology many months post-acute infection [20]. At least 20–45 percent of COVID-19 survivors experience various neuropsychiatric [14,21,22,23,24,25,26,27,28,29,30,31,32,33], neurological [34,35,36,37,38,39,40,41,42] and neurodegenerative [37,43] issues, sleep disturbances [44], and cognitive deficits [28,29,30,31,32,33,45,46,47,48], especially brain fog [16,17,49,50,51,52,53,54,55]. The length of long COVID may depend on the persistence of some viral antigens [56] and the magnitude of continued inflammatory reactions to SARS-CoV-2 [57]. Long COVID has been considered as the “Next national health disaster” for the US [58] that could have a “$3.7 trillion economic impact rivaling the Great Depression” [59].

A systematic review of the literature using the MEDLINE data base (1 January 1990–1 January 2023) was conducted to identify peer-reviewed publications relevant to the diagnosis, pathogenesis and treatment of neuro-COVID using the search terms angiotensin-converting enzyme 2 (ACE2), blood-brain barrier (BBB), brain, chemokines, corona virus, COVID-19, cytokines, endothelial cells, fatigue, fog, inflammation, Long-COVID, mast cell, microglia, neuroinflammation, toll-like receptors, and vasculature. Emphasis was placed on publications reporting original data, especially in humans, even though reviews and papers using animal models were also included.

We advance the premise that brain perivascular inflammation is a critical pathogenetic factor in long COVID mostly due to SARS-CoV-2 activating brain mast cells and microglia resulting in the release of inflammatory, neurotoxic, and vasoactive mediators [60,61].

2. Long COVID Pathogenesis Is Unknown

The precise mechanism of long COVID pathogenesis has yet to be fully elucidated [62]. It is well-understood that SARS-CoV-2 enters cells through the coronavirus spike (S) protein binding to its cell surface receptor, ACE2 [63]. How SARS-CoV-2 enters the brain is not yet clearly known, but increased levels of cytokines were reported in the cerebrospinal fluid (CSF) of patients with COVID-19 [64,65]. The virus may enter the brain from the nose through the nasal neural mucosa [66] following the olfactory nerve tract [67] or via the gustatory–olfactory trigeminal pathway and cause BBB dysfunction.

Autopsy from a deceased infant with COVID-19 showed severe neuronal loss in the capillaries of the choroid plexus [68]. Another autopsy study detected choroid plexus morphological changes in the microglia [69,70]_, as well as neuronal necrosis and glial cell hyperplasia in the brain of a deceased patient with COVID-19 [71]. While the exact pathogenetic mechanisms [50] remain unclear, evidence points to the involvement of neuroinflammation [72,73,74], and neurovascular inflammation that can damage brain blood vessels [75,76] and brain cells [72,77,78].

Role of SARS-CoV-2 S Protein in the Nervous System Damage

The neurological issues of long COVID [79] may be attributed to the entry of SARS-CoV-2 into the brain [80], but the routes of viral entry are not yet clear [81]. The S protein is involved in the fusion of the viral membrane with the surface membrane of the host. The S trimer structure has three receptor-binding domains (RBD), while the post-fusion structure expresses N-linked glycans that may protect the immune system [82]. Recent evidence and our studies indicate that the spike protein can also directly activate microglia [83,84,85], leading to proinflammatory effects [86] and microglia–synapse elimination [87]. SARS-CoV-2 can also lead to brain vascular damage and endothelial dysfunction [88,89,90,91], BBB disruption [92,93,94,95,96,97,98] and reduced blood flow to the brain [99]. A recent study reported that neuroinflammation could disrupt the “blood-central nervous system (CNS) barrier” in a mouse model of multiple sclerosis (MS) that involved the interaction of inflammatory, endothelial, and mesenchymal pathways [100]. Perivascular inflammation with lymphocytic and microglial infiltration was noted in the brains of 52 deceased patients with COVID-19 [101]. A cross-sectional study identified BBB disruption along with increased microglial activation markers and increased B-cell responses against self and non-self-antigens [102]. Another study with 24 neuro-COVID patients also reported increased intrathecal immunoglobulin, neopterin, and neurofilament light chain (NfL) levels [103]. Apolipoprotein E4 (ApoE4) has been associated with COVID-19 [104,105,106] and with severe COVID-19 [100,101]. In fact, ApoE4 could possibly predict COVID-19 pathogenesis [107]. In particular, ApoErs429358 polymorphism was associated with an increased risk of COVID-19 infection [108]. Elevated ApoE4 levels were reported to also reflect BBB disruption and predict cognitive decline [109].

SARS-CoV-2 has been reported to activate toll-like receptors (TLRs) [110,111] leading to the release of immune molecules that could contribute to neurologic symptoms [112]. TLRs are important in recognizing viral particles and orchestrate innate immune responses. Viral activation of TLRs causes the release of inflammatory cytokines from immune cells [113]. Microglia have many receptors, including TLRs [114], and they are activated by damage-associated molecular patterns (DAMPs) and pathogen-associated molecular patterns (PAMPs) [114]. TLRs were recently shown to mediate COVID-19 pathogenesis [115]. One paper reported that SARS-CoV-2 envelope protein could produce inflammatory cytokines from mouse bone-marrow-derived macrophages via TLR2 activation, independent of viral entry [116]. Another study demonstrated that SARS-CoV-2 S protein could stimulate BV-2 microglia leading to the release of interleukin-1 beta (IL-1β), IL-6, and tumor necrosis factor-alpha (TNF-α) with increased expression of TLR4 [84]. Another paper reported that infection of HMC3 microglia also led to the release of IL-1β, IL-6, and TNF-α [83]. Moreover, activation of TLR4 increased the expression of ACE2 [117], further enhancing viral infectivity in an autocrine loop fashion. In fact, TLR4 has been considered as a therapeutic target for neurological complications associated with SARS-CoV-2 infection [118,119]. Increased levels of pro-inflammatory cytokines, especially IL-6, have been detected in the CSF of COVID-19 patients [65] and have been implicated in neurologic diseases associated with COVID-19 [64]. Our recent findings show that SARS-CoV-2 can stimulate human microglia to secrete distinct pro-inflammatory mediators via activation of different receptors: recombinant whole-length S protein results in secretion of IL-1β and chemokine (C-X-C motif) ligand 8 (CXCL8) not via activation of ACE2, but rather activation of TLR-4, while the recombinant receptor binding domain (RBD) of the S protein stimulates the release of IL-18, TNF-α, and S100B via ACE2 [120].

3. Microglia-Induced Neuroinflammation and Mental Health

Microglia are specialized macrophage-like immune cells of the CNS and constitute about 7 percent of non-neuronal cells in the brain [121]. It has been reported that one microglial cell serves 1 to 100 neuronal cells in various brain areas with different neuronal densities [121]. Microglia are important for CNS homeostasis both in health and disease states [122], especially neurodegenerative [123,124,125,126,127,128,129] and neuroinflammatory [122,128,130,131] diseases, including COVID-19 [83,132]. During neuroinflammatory response and brain homeostasis maintenance, microglia can change their numbers, morphological characteristics, molecular pattern, and functions [132]. Activated microglia release pro-inflammatory cytokines, free radicals, and fatty acid metabolites. Cytokines and chemokines released from activated microglia induce activation of astrocytes with additional release of proinflammatory mediators that further exacerbates neuroinflammatory response. Dysregulated microglia and T-cell interactions and microglial nodules in the perivascular compartment of the brain were associated with systemic inflammatory conditions in COVID-19 [133]. Microglial activation is significantly higher in the brain stem than in non-COVID cases. Further, COVID-19 cases without dementia show more microglial activation in the brain stem [134,135]. The neuroinflammatory response is indicated by the presence of microglial reactivity indicators such as CD68-positive ameboid microglia, ionized calcium binding adaptor molecule 1 (IBA1), and human leukocyte antigen-DR (HLA-DR) in COVID-19 [132,134]. COVID-19 shows more T lymphocytes and microthromboses in the lung associated with more microglial activation in the brain stem [135]. In other words, the long-term consequences of COVID-19 could be due to persistent inflammation rather than persistent viral replication [135]. SARS-CoV-2 induces neuropsychiatric and neurological disorders such as cognitive decline, depression, dizziness, delirium, and sleep disorders that lead to neuronal damage, neurodegenerative disorders, and dementia [136]. Thus, SARS-CoV-2 can cause BBB disruption and worsen neurodegenerative diseases such as Alzheimer’s disease (AD) and Parkinson’s disease, especially in aged people [136,137,138].

SARS-CoV-2 infection can cause dysregulation of the hypothalamic–pituitary–adrenal (HPA) axis [139], which may be the cause of the emotional changes observed during and after viral infection [140]. Several reports have shown the impact of the pandemic on acute and chronic mental health. Further, these studies also focused on the psycho-social factors and stress resilience of mental health and disease pathologies [141,142]. TLR4 contributes to the immune response and pathogenesis of COVID-19, and thus, TLR4 could be a therapeutic target in COVID-19 [113,143,144]. SARS CoV-2 activates TLR4 and 8 and induces cytokine release from microglia and monocytes [145]. Microglia express receptors for neurotensin (NT) [146] and corticotropin-releasing hormone (CRH), secreted under stress [147], which are especially associated with COVID-19 [148]. Microglia are typically characterized as resting (M0), pro-inflammatory (M1), and anti-inflammatory and neuroprotective (M2) phenotypes with different cytokine expressions associated with neuroinflammatory response. We reported that cultured human microglia can be activated by neuropeptides such as NT to release IL-1β and CXCL8 [149] that induces proinflammatory response. Microglial-derived proinflammatory cytokines and chemokines induce astrogliosis, amyloid deposition, and subsequently, further worsening neuroinflammation [122]. Psychological stress can increase microglial reactivity to other challenges [150] and lead to cognitive decline [151] and neuroinflammatory response.

Microglia are increasingly involved in the pathogenesis of psychiatric disorders [132,152,153]. In fact, microglia-induced neuroinflammation was considered a risk factor for the pathogenesis of major depressive disorder [154,155]. Moreover, SARS-CoV-2 neurotropism may increase the severity of neuropsychiatric issues [156]. A recent report indicated that the SARS-CoV-2 protein elicited a robust nuclear factor kappa B (NF-κB)/nucleotide-binding domain (NOD)-like receptor protein 3 (NLRP3) inflammasome-mediated pro-inflammatory response and increased Iba1 expression in a BV-2 mouse microglial cell line [84]. In addition, post-mortem reports of COVID-19 patients showed significant microglial activation and neuroinflammation associated with brain pathology [157,158,159,160]. Increasing reports indicate that elevated inflammatory cytokines and neuroinflammatory responses [72,128,161] can damage brain blood vessels [75,162] and other brain cells [72,77,78], possibly through abnormally excessive activation of microglia [60,61]. As such, long COVID could be referred to as “brain autoimmunity” [163].

4. Microglia Communicate with Mast Cells

Mast cells communicate with microglia [32,164], leading to their activation [33,164,165,166] and contributing to neuroinflammation [32,33] and neurodegenerative diseases [32,167]. This effect is not seen in mast-cell-deficient mice [168,169]. In fact, mast cell proteases can trigger astrocytes and glia/neurons and release IL-33 [170]. Stabilization of mast cells was shown to inhibit lipopolysaccharide (LPS)-induced neuroinflammation by suppressing the activation of microglia [171]. Activation of mast cells and microglia in the hypothalamus and brain [172] could lead to cognitive dysfunction [173] and neuronal apoptosis (Figure 1) [173]. In addition, mast cells can activate the hypothalamic–pituitary–adrenal (HPA) axis [174,175,176,177] through the release of histamine [178], IL-6 [179], and CRH [180]. It is interesting that stress has been linked to the possible priming of immune cells thus contributing to neuroinflammation in AD [181,181]. Furthermore, NT [182,183] and substance P (SP) [2] induce CRH receptor-1 (CRHR1) expression in mast cells. Mast-cell-derived histamine [184] and tryptase [185] can trigger microglia and induce neuroinflammation [33]. Mast cells have been shown to be an early activator of LPS-induced neuroinflammation and BBB damage in the hippocampus [172]. In addition, food allergy that depends on mast cell activation has been shown to increase activated microglia and TNF in the hippocampus, associated with behavioral and learning impairments [186]. Another paper reported that early stress in mice and humans disrupted interactions between mast cells and glia via the involvement of histamine [187]. As such, mast cells can participate in neuroinflammation [188,189] by releasing histamine and several inflammatory cytokines and chemokines [190].

Figure 1.

Schematic diagram showing the proposed role of mast cells and microglia in the pathogenesis of neuro-COVID. A variety of triggers including toxins and viruses such as SARS-CoV-2 can reach the hypothalamus, mostly through the nose and olfactory nerve tract. There, they can disrupt the BBB via activation of perivascular mast cells, which then further increase BBB permeability and activate microglia. Proinflammatory molecules released from microglia can damage neurons, disrupt homeostasis, and contribute to the pathogenesis of neuro-COVID.

5. Mast Cells in the CNS

Mast cells are ubiquitous in the body [191]. They are mostly known for mediating allergic and anaphylactic reactions [192], and several other diseases such as mastocytosis [193]. The functions of mast cells in health and several pathologic conditions were reviewed recently [194,195,196,197]. Mast cells respond to allergic but also to various other non-allergic stimuli [193]. Activated mast cells can secrete as many as 100 biologically powerful mediators, including pro-inflammatory molecules [190] such as bradykinin, chymase, histamine, tryptase, chemokine (C-C motif) ligand 2 (CCL2), CXCL8 [198], IL-6 [199], IL-1β, and TNF-α [200]. A particular potent stimulus of the mast cells is the peptide SP, especially when primed by the “alarmin” cytokine IL-33 [201,202,203,204]. In addition, we showed that SP can induce expression of the IL-33 receptor (ST2) [200], thus further increasing mast cell stimulation. Mast cells can also be stimulated to secrete mitochondrial DNA (mtDNA) [205], which serves as an additional “alarmin” and can trigger an auto-inflammatory reaction [206,207]. Mast cells are also found in the CNS perivascularly [29,208], especially in the meninges [28,209] and the median eminence of the hypothalamus [122,209,210], where they could have numerous functions (Table 1). We have called brain mast cells the “immune gate to the brain” [29]. Functional interactions have been reported between mast cells and neurons [209,211] that are often positive for CRH [183,209]. Mast cells are the richest source of histamine in the CNS, particularly in the amygdala, hippocampus, hypothalamus, and thalamus [212,213]. Stimulated brain mast cells contribute to postoperative cognitive dysfunction (POCD) through the release of inflammatory and neurotoxic mediators from activated microglia [86,173]. Activation of mast cells [183] and microglia in the hypothalamus [49] could cause cognitive dysfunction [173] that is also seen in patients with mastocytosis [47,214,215] and may be related to brain abnormalities [216]. Allergic stimulation of nasal mast cells resulted in stimulation of the HPA axis [174,175,176,177], possibly via mast cell release of histamine [178], IL-6 [178,217], and CRH [180]. The influence of stress on mast cells has also been reviewed [140,218]. Restraint stress in rodents increased BBB permeability [210,219,220] via CRH [219,221,222]. Mast-cell-released cytokines [223,224] increased BBB permeability [210,219] and permitted mammary adenocarcinoma brain metastases in mice [221]. This process could worsen with stress, acting via CRH stimulation of mast cells [219,221] and an increase in dura vascular permeability. Meningeal mast cells affected the integrity of the BBB and promoted T-cell brain infiltration [225]. Inflammation mediated by mast cells and microglia disrupted the BBB [226]. Mast cell responsiveness may be regulated not only by the neuroimmune stimuli but also by the effects of the different receptors involved. For instance, mast cells express high-affinity neurokinin-1 (NK-1) receptors for SP [2]. Moreover, SP and NT [182] induced the expression of CRHR-1 in human mast cells. Secretion of mediators can occur by utilizing different signaling [227,228,229,230] and secretory [228,230] pathways. The regulation of mast cells by neurotransmitters and neuropeptides has been reviewed [231,232,233], with emphasis on CRH [177], hemokinin-1 (HK-1) [234], nerve growth factor (NGF [235], NT [236], SP [237], and somatostatin [238,239] acting via activation of high-affinity receptors. Activated mast cells could release a number of pro-inflammatory and vasoactive mediators that could contribute to long COVID syndrome symptoms [177,240]. Some mediators are pre-stored in secretory granules (e.g., histamine, tryptase, and TNF-α) [241,242] and are released immediately following stimulation, while others are newly synthesized and then released, such as chemokines (e.g., CCL2, CCXL8) [198], and cytokines (IL-6 [199], IL-1β [243], TNF-α [200]). Apart from allergic triggers acting via IgE, mast cells are stimulated by non-allergic agents [192,203,244], especially neuropeptides [231], such as SP [237,243] and the SP-related HK-1 [234], which have pro-inflammatory properties. Under such conditions, especially when primed by IL-33 [203,204], mast cells can release various inflammatory mediators without the release of histamine or tryptase [245], thus contributing to inflammatory disorders [189,192]. Moreover, mouse mast-cell proteases 6 (MMCP 6) and MMCP 7 stimulated the release of IL-33 from mouse fetal-brain-derived cultured primary astrocytes in vitro [170]. A case in point is the selective release of IL-6 [199,246], which is elevated in systemic mastocytosis and correlated with disease severity [247,248,249] and can increase mast cell numbers [250].

Table 1.

Mast cell actions associated with brain pathophysiology.

Angiogenesis

Activation of microglia, astrocytes, and neurons

Cognitive decline

Disruption of the BBB and entry of peripheral inflammatory mediators, cells, and pathogens/SARS CoV-2 into the brain parenchyma

Early responders in brain injury

Growth factor secretion

Increase vascular permeability

Interactions with microglia, astrocytes, and neurons

Neuroprotection

Neurodegeneration

Neuroinflammation

Proinflammatory mediator release

Posttraumatic stress disorder (PTSD)

Regulation of the HPA axis and stress response

Secretion of proinflammatory, neurotoxic, and vasoactive mediators

Vascular permeability

6. Mast Cells in Long COVID

Mast cells are activated by viruses [251,252] such as SARS-CoV-2 [17,18,20,53,55,57,253,254,255,256,257,258,259,260,261]. Recent studies have also reported mast cell activation in the lungs [254] and perivascular inflammation in the brains [75] of COVID-19 patients. We hypothesized that the spike protein can get into the brain either directly or through the activation of mast cells, which then disrupts the integrity of the BBB (Figure 1) [79]. Two studies reported elevated serum levels of chymase in patients with COVID-19 [253,260]. Moreover, a recent study demonstrated that mast cells enhance cellular entry of SARS-CoV-2 through the generation of chymase-spike complexes [52]. Chymase converts angiotensin I to angiotensin II and may act in an autocrine fashion to increase the expression of ACE2, which then facilitate viral entry. Another paper reported that mast-cell-derived histamine can increase SARS-CoV-2 entry into endothelial cells [90]. Mast cells also release extracellular mtDNA [205], which was shown to be significantly elevated in COVID-19 patients [262]. Extracellular mtDNA can then stimulate the secretion of pro-inflammatory mediators from other immunocytes [206,207].

7. Neuroimmune Biomarkers

While a number of molecules are elevated in the blood of patients with COVID-19 [34,35,36,263], the results have been inconsistent and have focused primarily on pro-inflammatory mediators. A few studies have investigated blood biomarkers that may reflect brain injury in COVID-19 patients [264,265]. Anti-receptor antibodies and autoimmune gene expression [266] have also been reported. IL-15 is implicated in viral clearance with anti-viral properties, including in COVID-19 [267,268]. We showed elevated IL-18 in the serum of patients with COVID-19 [269]. IL-18 remains elevated longer than other cytokines in inflammatory and autoimmune disorders [270,271], including COVID-19 [269]. Calprotectin (S100A8/A9) was associated with microglia activation [272] and was elevated in the serum of patients with COVID-19 [269]. Calprotectin was also in the CSF of patients with Multiple Sclerosis (MS) [273] and demyelinating polyneuropathy [274]. Neuroligins (NLGs) and neurexins are implicated in synaptic function and cognitive disease [275]. NLG1 levels were reduced in the cortex and the CSF of AD patients [276] or those with mild cognitive impairment (MCI) [277]. NLG4 was associated with cognitive decline [278], while neuropilin-1 (NRP-1) was shown to facilitate SARS-CoV-2 entry by binding to the spike protein [279]. Moreover, S100β was shown to be associated with COVID-19 severity [280] and promote microglia activation [281,282,283] and has been linked to neuroinflammation and cognitive decline [284]. Neurofilament light chain (NfL), microtubule-associated protein-2 (MAP-2), and glial fibrillary acidic protein (GFAP) indicate axonal/neuronal damage and brain injury [264,285,286,287,288]. Elevated levels of osteopontin have been associated with reduced cognition [289,290]. A recent study indicated that COVID-19 was associated with brain pathology in the UK Biobank [291] and was associated with neuroinflammation involving primarily the chemokine CCL11 in a mouse model [292]. CCL11 has been implicated in neuroinflammatory disorders [293], while osteopontin was reported to disrupt the BBB [294]. Chemokine CCL19 and its receptor C-C chemokine receptor type 7 (CCR7) axis are involved in the immune response to viral infections [268,295]. Increased levels of CCL19 were associated with disease severity in COVID-19 patients [296].

8. Lack of Effective Treatments

To date, there are no effective drugs to either treat long COVID or mitigate the release of inflammatory mediators from microglia. Understanding how neuro-immune and toxic triggers contribute to long COVID and how to regulate this response is of clinical importance (Figure 2). One of the major impediments has been the lack of appropriate disease surrogates either in vivo or in vitro [297], as well as the lack of effective inhibitors of neuroinflammation. Apparently, there have been therapeutic considerations of “stabilizing” the BBB [226,298].

Figure 2.

Schematic diagram showing the proposed beneficial effects of eriodictyol. Neuro-COVID can activate mast cells, and several inflammatory mediators released from activated mast cells can activate microglia and other brain cells to release inflammatory and neurotoxic mediators that can cause neuroinflammation and neurodegeneration and exacerbate neuro-COVID disease severity. Eriodictyol could inhibit neuro-COVID-associated mast cell activation-mediated inflammatory mediator release as well as inflammatory mediators released from activated microglia. These inhibitions could reduce the disease severity or treat neuro-COVID.

For inflammation, non-steroidal anti-inflammatory drugs (NSAIDs) did not improve COVID-19 [297]. Biologics have also been tried in COVID-19. Even though IL-6 has been reported to be elevated and possibly an independent risk factor, clinical trials using IL-6 inhibitors did not show any consistent benefit in COVID-19 [299]. One study reported that a clinically available IL-1β antagonist significantly improved COVID-19 with secondary hemophagocytic lymphohistocytosis (sHLH) that was characterized by pancytopenia, hyper-coagulation, and acute kidney injury [300]. Glucocorticoids have been used extensively in severe, hospitalized patients with COVID-19 [301], but the results are confusing. One paper reported a reduction in mechanical ventilation and a 20 percent reduction in the mortality rate of COVID-19 patients but also longer hospital stays and longer viral clearance time [302]. A more recent systematic review and meta-analysis showed a trend toward a higher discharge rate, but the effect was minimal and not significant [301]. Another analysis of 16 randomized control trials reported that systemic corticosteroids slightly reduced 30-day mortality in severe patients, but there was no benefit up to 120 days [303]. A multicenter observational cohort study conducted in 55 Spanish intensive care units reported that early administration of high doses of dexamethasone since symptom onset could actually prove harmful for 90-day mortality [304]. In fact, it has been argued that even though glucocorticosteroids may improve outcomes in severe, intubated patients with COVID-19, they could also reduce the production of antiviral IgG antibodies [305], thus hampering protection from other infections and worsening long-term outcomes [306].

Inhibition of brain inflammation could instead be accomplished with the use of some natural flavonoids [79,307,308,309,310,311,312]. In particular, the flavone luteolin inhibits both microglia [149,313,314] and mast cells [315,316], as well as related inflammatory processes [147,311]. A novel luteolin analogue, tetramethoxyluteolin [147], can inhibit secretion of the cytokines IL-1β and TNF-α [149], as well as the chemokines CCL2 and CCL5 [198], from human microglia [149,314] and mast cells [220,299]. Flavonoids have been reported to prevent neuroinflammation [311,312,317,318], provide neuroprotection [311,318,319,320,321], and reduce cognitive dysfunction [322,323,324,325,326], especially brain fog [48,327,328]. However, flavonoids are difficult to dissolve in aqueous solutions and also have poor oral absorption and bioavailability. Two formulations containing liposomal luteolin (BrainGain^® and FibroProtek^® were successfully used to treat a severe COVID-19 patient with brain fog [329]. We have identified a novel flavonoid that is structurally similar to luteolin, the flavanone eriodictyol [330,331,332], which is also partially water-soluble and may be particularly suited for development as an effective treatment (Figure 2) because of its multiple beneficial actions (Table 2) [333,334,335]. A new and novel dietary supplement (ViralProtek^®) combines eriodictyol [334,335,336,337] with oleuropein from olive leaves [338,339,340] and sulforaphane from broccoli [341] and was recently shown to have strong coronavirus inhibitory properties.

Table 2.

Beneficial actions of eriodictyol.

Ameliorates cognitive dysfunctions

ACE2-RBD blocker

Anti-inflammatory

Antioxidant

Cardioprotective

Hepatoprotective

Inhibits brain injury and neurological deficits and improves memory impairment

Inhibits synaptic dysfunctions

Inhibits oxidative stress-associated cell death

Inhibits stress-induced deleterious effects

Neuroprotective

RNA polymerase inhibitor

SARS-CoV-2 protease inhibitor

9. Conclusions

Neuro-COVID is a common presentation of long COVID patients and could be at least partly caused by the activation of brain mast cells and microglia, leading to perivascular inflammation and disruption of neuronal connectivity and neuronal signal transmission. In the absence of any approved drugs, a combination of certain natural compounds could help minimize these processes and associated symptoms.

Abbreviations

AD	Alzheimer’s disease
ACE2	Angiotensin-converting enzyme 2
Apoe4	Apolipoprotein E4
BBB	Blood–brain barrier
CNS	Central nervous system
CSF	Cerebrospinal fluid
CXCL8	Chemokine (C-X-C motif) ligand 8
COVID-19	Coronavirus disease-2019
CRH	Corticotropin-releasing hormone
DAMPs	Damage-associated molecular patterns (DAMPs)
GFAP	Glial fibrillary acidic protein
HK-1	Hemokinin-1
HLA	Human leukocyte antigen
Iba1	Calcium-binding adapter molecule 1
IL-1β	Interleukin-1 beta
LPS	Lipopolysaccharide
MMCP6, 7	Mast cell proteases 6 and 7
MAP-2	Microtubule-associated protein-2
MCI	Mild cognitive impairment
mtDNA	Mitochondrial DNA
MS	Multiple sclerosis
NGF	Nerve growth factor
NfL	Neurofilament light chain
NK-1	Neurokinin-1
NLGs	Neuroligins
NT	Neurotensin
NSAIDs	Non-steroidal anti-inflammatory drugs
NF-κB	Nuclear factor kappa B
NLRP3	Nucleotide-binding domain (NOD)-like receptor protein 3
PAMPs	Pathogen-associated molecular patterns
PD	Parkinson’s disease
PASC	Post-acute sequelae of SARS-CoV-2
PTSD	Post-traumatic stress disorder
S	Spike
SARS-CoV-3	Severe Acute respiratory syndrome coronavirus 2
SP	Substance P
TLRs	Toll-like receptors
TNF	Tumor necrosis factor

Author Contributions

Conceptualization, original manuscript preparation, and review & editing, T.C.T.; review and editing, D.K. All authors have read and agreed to the published version of the manuscript.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Not applicable.

Conflicts of Interest

The authors declare no conflict of interest.

Funding Statement

This research received no external funding.