Abbreviations
ACE2: Angiotensin-converting enzyme 2 receptor
α7nAChR: Alpha 7 acetylcholine nicotinic receptor
Ca2+/CaMKII:
Ca2+/calmodulin-dependent protein kinase II
CaM: Calmodulin
IKK: I-KappaB-alpha kinase complex
IL-1β: Interleukin 1β
IL-6: Interleukin 6
JAK2: Janus kinase 2
MT1/MT2: Melatonin receptor subtypes 1 and 2
NF-κβ: Nuclear factor kappa-light-chain-enhancer of activated B cells
NLRP3: NLR pyrin domain containing 3
ROS: Reactive oxygen species
SARS-CoV-2: Severe acute respiratory syndrome coronavirus 2
STAT3: Signal transducers and activators of transcription
TLR: Toll like receptor
TMPRSS2: Transmembrane serine protease 2
TNF-α: Tumor necrosis factor-α
TNFR: TNF receptor
TRADD: TNF receptor-associated death domain
TRAF2: TNF receptor-associated factor-2
1. Introduction
In late December 2019, a number of 27 people with clinical symptoms of
dry cough, dyspnea, fever, and bilateral lung infiltrates on imaging
with an unknown cause were diagnosed in Wuhan, Hubei Province, China. A
few days later, a new virus of great global public health concern was
isolated and designated as severe acute respiratory syndrome coronavirus
2 (SARS-CoV-2), later referred to as coronavirus disease-2019 (COVID-19)
(Chen et al., 2020). This new virus was found to be highly contagious
and quickly spread globally, leading the World Health Organization (WHO)
to officially declare the COVID-19 outbreak a global pandemic on March
11, 2020 (Mahase, 2020). As an emerging acute respiratory infectious
disease, clinical symptoms are characterized predominantly by fever and
dry cough followed by dyspnea, bilateral infiltrates, sputum production,
haemoptysis, lymphopenia, shortness of breath, sore throat, neurological
and gastrointestinal manifestations (Huang et al., 2020; Wang et al.,
2020). COVID-19 employs cell receptor angiotensin-converting enzyme 2
(ACE2) for host cell entry (Xu et al., 2020; Zhou et al., 2020), which
is present in the type II alveolar cells (Zou et al., 2020), enterocytes
from ileum and colon (Xiao et al., 2020), liver cholangiocytes (Qi et
al., 2020), aqueous humor (Holappa et al., 2015), myocardial cells
(Donoghue et al., 2000), proximal tubule kidney cells (Zou et al.,
2020), urothelial bladder cells (Zou et al., 2020), epithelial cells of
the oral mucosa (Xu et al., 2020), as well as neurons and glia in the
brain stem and particular cerebral regions (Xia & Lazartigues, 2010).
The high expression and wide distribution of ACE2 receptor in human body
may sustain the ubiquitous potential infection of COVID-19 and explain
its tropism. It is also recognized that COVID-19 could be transmitted
via multiple routes, predominantly binding to ACE2 alveolar epithelial
cells (Wan et al., 2020; Zhou et al., 2020) or by oral-fecal
transmission (Gu et al., 2020), among others. To date, it is accepted
that the incubation period of COVID-19 is up to 14 days, although it has
been suggested that it may be extended up to 24 days, which possibly
reflects a double exposure. Furthermore, asymptomatic infection has been
additionally reported (Huang et al., 2020; Linton et al., 2020).
Interestingly, previous studies showed that the coronavirus also infects
the central nervous system (CNS) (Arbour et al., 2000; Lau et al., 2004)
since it can spread from the respiratory tract to the CNS, showing
neuroinvasive capacities.
There is uncertainty about extra-pulmonary manifestations of COVID-19,
including those affecting the CNS. Numerous efforts to implement
effective therapeutic strategies are underway. The distressing scenario
that the world is experiencing regarding COVID-19 calls for new
treatment approaches and, in this respect, we want to emphasize the
advantages of melatonin (N -acetyl-5-methoxytrytamine) as a
potential therapeutic agent to ameliorate the CNS damage associated with
SARS-CoV-2’s disease. Melatonin is not a molecule addressed to diminish
the viral load nor target specific enzymes involved in viral replication
and transcription, nevertheless it has multiple indirect anti-viral
actions (Anderson et al., 2015; Elmahallawy et al., 2015; Junaid et al.,
2020; Montiel et al., 2015; Tan et al., 2014). In this regard, two
recent reviews have suggested the utility of melatonin as adjunctive or
even regular therapy for COVID-19 patients who suffer pneumonia, acute
lung injury (ALI) and acute respiratory distress syndrome (ARDS) (Tan &
Hardeland, 2020; Zhang et al., 2020). Regarding the experimental
clinical use of melatonin in the current critical situation, it is
important to highlight that melatonin is not only a well-known
anti-inflammatory (Carrascal et al., 2018), anti-oxidative (Rodriguez et
al., 2004), and immune-enhancing agent (Carrillo-Vico et al., 2005) but
also a molecule with a high safety profile (Seabra et al., 2000). Its
small size and amphiphilic nature allow melatonin high cell diffusion
capability and it has high permeability through biological compartments,
including blood brain barrier (BBB) (Tarocco et al., 2019), reaching
cytosolic, mitochondrial, and nuclear compartments (Menendez-Pelaez &
Reiter, 1993; Reiter et al., 2020b). BBB integrity is crucial for the
maintenance of CNS and thus several neurological disorders debut after
deterioration of the BBB (Rosenberg, 2012). Melatonin restores BBB
homeostasis limiting microvascular hyperpermeability (Alluri et al.,
2016; Liu et al., 2017) and therefore making it a promising candidate
against neuroinvasion caused by COVID-19. In view of all above
information, this review focuses on characteristics, mechanisms, and
implications of CNS involvement caused by SARS-CoV-2 infection, and we
speculate how the use of the melatonin could become a basis for a
possible neurotherapeutic approach.
2. How does SARS-CoV-2 neuroinvasiveness occur? Melatonin potential to
respond to COVID-19 neuropathogenesis
SARS-CoV-2 shares its clinical symptoms with those described for
SARS-CoV and Middle East respiratory syndrome CoV (MERS-CoV)
betacoronaviruses. Previously, it has been reported the presence of
SARS‐CoV and MERS-CoV particles in the brain of experimental animal
models and patients and (Ding et al., 2004; Li et al., 2016; Xu et al.,
2005). However, regarding SARS‐CoV‐2 two important questions that arises
are: i ) The exact route by which the virus enters the CNS andii ) whether its the presence may be detected in the cerebrospinal
fluid (CSF) or postmortem tissue specimens collected from COVID-19
victims. It is known that human coronaviruses can penetrate the CNS
through different routes, directly from the external environment
affecting the olfactory nerve and olfactory bulb neuroepithelium or
using either the lymphatic or the hematogenous route, which represents
an excellent opportunity to infect endothelial cells of the BBB
(Desforges et al., 2014). It has been recently reported that human ACE2
receptors are the gateway for SARS-CoV-2 and that transmembrane protease
serine 2 (TMPRSS2) is essential for the SARS-CoV-2´s spike (S) protein
activation, which facilitates viral attachment to the surface of target
cells (Hoffmann et al., 2020). Consequently, it can be thought that
those brain regions with the highest expression of ACE2 receptors could
turn out to be more affected by SARS-CoV-2 infection, although the
involvement of other receptors or co-receptors is not definitely
discarded. In this context, a recent study using mRNA expression levels
data provided by the Allen Human Brain Atlas has demonstrated ACE2 in
brain regions, such as olfactory bulb (OB) (Lapina et al., 2020). This
may explain the neuropathogenic impact of SARS-CoV-2 on the partial loss
of the sense of smell or total anosmia, as a result of nasal
inflammation, mucosal edema, and obstruction of airflow into the
olfactory cleft. Additionally, the analyses of three databases have
shown high expression levels of human ACE2 in regions of the human brain
(e.g., substantia nigra) and both neurons and astrocytes, which would
help us to understand how SARS-CoV-2 can spread into the brain and cause
neurological damage (Chen et al., 2020) (Figure 1). Conversely, OB
imaging by magnetic resonance was found normal in a COVID-19 patient
with anosmia and without intensity signal of nasal congestion in the
early phase of the disease (Galougahi et al., 2020); even though,
anosmia may persist for a long time. Another essential point for initial
viral neuroinvasion is the recent announcement by genome sequencing of
the first reported case of SARS-CoV-2 in the CSF (Xiang, 2020).
Additionally, a recent case study reporting viral particles in
endothelial cells and frontal lobe sections obtained at forensic
examination confirmed the presence of SARS-CoV-2 in neural tissue
(Paniz-Mondolfi et al., 2020). After these observations, brain infection
is being seriously considered by the scientific community because
important pathologies at neurological level are emerging associated with
COVID-19 patients; encephalopathy (Filatov et al., 2020; Poyiadji et
al.), meningitis/encephalitis (Moriguchi et al., 2020; Ye et al., 2020),
Guillain-Barré syndrome (Zhao et al., 2020), cerebrovascular disease
(Helms et al., 2020; Li et al., 2020; Wu et al., 2020) and epilepsy
(Karimi et al., 2020). Moreover, Li and colleagues (Li et al., 2020)
have hypothesized that the ability of SARS-CoV-2 targeting the CNS may,
at least in part, explain the acute respiratory failure of COVID-19
patients. Conversely, it has been argued that the brain dysfunction
induced by SARS-CoV-2 still lacks strong evidence and, therefore, this
relevant question should be further investigated.
Taken together, the abovementioned evidence indicates that counteracting
or mitigating the neuroinvasion of SARS-CoV-2 emerges as an essential
strategy to prevent or treat COVID-19. At this point is where melatonin
can act as a protective agent against virus-related diseases (Anderson
& Reiter, 2020; Elmahallawy et al., 2015; Silvestri & Rossi, 2013; Tan
et al., 2014) and several pathologies of CNS ((Brigo et al., 2016; Farez
et al., 2015; Gerber et al., 2005; Ramos et al., 2017) including those
affecting BBB (Ramos et al., 2017) (Figure 1). Regardless, is it
possible that melatonin acts inside the neural cells targeted by
SARS-CoV-2. Its amphiphilic nature helps it to reach intracellular
organelles, binding to mitochondrial and cell cytosol proteins,
increasing, thus, its neural availability (Tan, 2010) and therapeutic
versatility in at least three different ways. Firstly, melatonin binds
to calmodulin (CaM) and may act on the
Ca2+/calmodulin-dependent protein kinase II
(Ca2+/CaMKII) system, thereby regulating the
expression of ACE2 (Lambert et al., 2008). Secondly, CaMKII has been
found to copurify with proteasomes of the brain (Bingol et al., 2010)
and the ubiquitin-proteasome system is involved in the early viral
replicative cycle. Interestingly, like a proteasome inhibitor (Vriend &
Reiter, 2014), melatonin may regulate several events involved in
proteostasis, through the Ca2+/CaMKII system, which
can also influence SARS-CoV-2 infectivity. Thirdly, given its tectonic
impact in host cell homeostasis, it is expected that SARS-CoV-2
dysregulates mitochondrial metabolism. In this regard, it is worth
noting that many of the actions displayed by melatonin are directed to
maintain mitochondrial function. Therefore, any decrease of melatonin
(pineal or mitochondrial) levels may open a way for depleting the viral
control of cellular metabolism and thus slowing down the replication of
SARS-CoV-2 (Anderson & Reiter, 2020).
About 10-15% of the COVID-19 patients with ARDS and organ failure have
been associated with the commonly known as hyperinflammation or
“cytokine storm syndrome”. The question that arises is; might
SARS-CoV-2 infection trigger a cytokine storm in the brain? In light of
recent COVID-19-associated case reports describing a rare form of severe
brain damage with hemophagocytic lymphohistiocytosis (HLH) (Radmanesh et
al., 2020), and a patient with an acute necrotizing hemorrhagic
encephalopathy (Poyiadji et al., 2020), it should be considered that
COVID-19 infection may induce a “cytokine storm syndrome” in the brain
(Mehta et al., 2020), deserving special consideration and further
research. In agreement with the above, it has also been reported that
up-regulation of proinflammatory mediators and a deregulated immune
response can be useful predictors of lethality by COVID-19 (Ruan et al.,
2020). Therefore, there are a plethora of alterations at interconnected
signaling pathways, such as the mammalian target of rapamycin (mTOR)
(Zhou et al., 2020), sirtuins (SIRTs) (Anderson & Reiter, 2020), NLRP3
inflammasome (Deftereos et al., 2020), Toll Like Receptors (TLRs) (Conti
et al., 2020) or single-pass type I transmembrane receptor (Notch)
(Rizzo et al., 2020) are key factors that may modulate COVID-19-related
cellular and molecular events (Figure 2). In this context, as an
universal regulator targeting a large number of different signaling
pathways and physiological processes, melatonin may exerts a significant
neuroimmunomodulatory protection against viral infections (Liu et al.,
2019; Ma et al., 2018; Shukla et al., 2019; Tiong et al., 2019; Xu et
al., 2018). Therefore, the capacity of the indoleamine for counteracting
the neuroinvasion by SARS‐CoV‐2-infection cannot be underestimated, nor
neglected and unquestionably requires further research.
3. Melatonin receptors in the context of CNS involvement for SARS-CoV-2
Given the astonishing pleiotropy of melatonin, it may be expected that
cellular and molecular mechanisms by which this indoleamine mediates
neuroprotection would be complex as well. To a some extent the
biological effects of melatonin are mediated through the interaction
with two high-affinity G protein-coupled receptors, MT1 and MT2, which
are involved in multiple signaling cascades in cell protection and
survival (Liu et al., 2016). Both in the CNS and peripheral organs
melatonin receptors are ubiquitously distributed (Ekmekcioglu, 2006; Ng
et al., 2017). Unfortunately, we must assume our current ignorance of
the mechanisms by which MT1 and MT2 may influence melatonin-mediated
signaling in the brain of patients afflicted by COVID-19. However,
melatonin receptor‐mediated protection has already been suggested
against lethal viral diseases such as the Venezuelan equine
encephalomyelitis (VEE) virus (Valero et al., 2009) or to improve the
total antioxidative defense capacity against respiratory syncytial virus
(RSV) (Huang et al., 2010). Moreover, it has been reported that 1/3
patients with confirmed COVID-19 present acute cerebrovascular disease
and epilepsy, among other neurological symptoms (Jin et al., 2020). It
is also known that COVID-19 patients might develop a “cytokine storm
syndrome” due to an exacerbated level of pro-inflammatory biomarkers
which contribute to significantly increase the risk of ischemic stroke.
SARS-CoV-2 may enter the CNS via the haematogenous diffusion and infect
the endothelial cells of the BBB. In this context, melatonin may elicit
part of its neuroprotective effect through melatonin receptors, thus MT2
up-regulation would not only preserve BBB integrity but also attenuate
the activation of astrocytes and microglia (Lee et al., 2010).
In Wuhan, a case of epilepsy in a COVID-19 patient was reported (Mao et
al., 2020). Indeed, down-regulation of the hippocampal MT2 receptor
conferred protection against seizures and exhibited the anticonvulsant
activity of melatonin (Stewart & Leung, 2005). Additionally, T
lymphocytes seem to be more vulnerable against SARS-CoV-2 infection
through S protein and CD147 (Wang et al., 2020), an extracellular matrix
metalloproteinase inducer of the proinflammatory cytokine cyclophilin A,
secreted by monocytes/macrophages and endothelial cells. T lymphocytes
express both melatonin receptors, MT1 and MT2 (Pozo et al., 2004;
Slominski et al., 2012), and in this regard, in the infected T cells
melatonin may; i ) regulate the immunostimulatory activity
mediated by MT1 and MT2 and ii ) block cyclophilin A/CD147
signaling pathway (Su et al., 2016). For these reasons, in the search
for new pharmacological strategies against COVID-19, we focused our
attention on the exogenous supplementation with melatonin to preserve
the immune response and counteract the neuroinflammation through its
widely distributed receptors in the CNS and most of the immune cell.
4. Understanding of COVID-19’s long-term impact on the CNS and
the influence of melatonin as a preventive
agent
All countries are suffering the scourge of COVID-19. Unfortunately,
science is still learning the natural history and pathogenicity of this
emerging coronavirus as well as its immediate and devastating effects on
human health. We should ask ourselves, what will be the long-term
consequences of COVID-19? Lippi and co-workers (Lippi et al., 2020) have
proposed the possibility that months or years after infection several
tissues, including the brain, patients may suffer an accelerated aging,
which could manifest in neurodegenerative disorders such as Parkinson
disease. Something that seems epidemiologically proven is that the most
susceptible people to SARS-CoV-2 infection are the middle-aged and
elderly. Based on this clinical evidence, we hypothesize that, at least
partially, the progressive melatonin decline with age may account for
the apparent increased COVID-19 sensitivity over life-span and specially
in the elderly. As a consequence of aging, the pineal gland accumulates
calcium deposits and both serum and CSF melatonin release decreases
(Reiter et al., 1981; Reiter et al., 2014), which is being increasingly
related with numerous dysfunctions and pathophysiological changes
(Karasek, 2004). The involvement of melatonin in the context of
neurodegeneration is promising since it has been well documented to
counteract most of the physiopathological events that trigger
neurodegenerative disorders (Ramos et al., 2020). Interestingly, a
certain relationship between nicotinic acetylcholine receptors (nAChRs)
and infectivity by SARS-CoV-2 is suspected. However, this raises the
controversy about whether it is a facilitating effect because nicotine
would promote SARS-CoV-2 cell penetration through nAChRs up-regulation
(nAChRs) (Kabbani & Olds, 2020) or the opposite by the low prevalence
of smokers among COVID-19 people in China (Guan et al., 2020) or the
hypothesized competition of nicotine and SARS-CoV-2 for binding nAChR
(Changeux et al., 2020). Thereby, a putative role of nAChRs in the
modulation of ACE2 has been suggested at cardiovascular level (Oakes et
al., 2018) and presently it should be considered a possible interplay
between both ACE2 and overexpressed nAChRs in the context of the
SARS-CoV-2 neurotropism and neuroinfection. At this point, melatonin
becomes relevant by the modulation of neuroinflammation (Niranjan et
al., 2012) and oxidative stress (Parada et al., 2014) via alpha 7
acetylcholine nicotinic receptor (α7nAChR) which oligomers are among the
most frequent in the brain (Gotti & Clementi, 2004) and mediate many of
the beneficial actions of the indoleamine including mitochondrial
regulation. It is also important to note that the COVID-19 may impact on
the neurotransmission process. Nataf (Nataf, 2020) using a
multiexperiment matrix showed a significant co-expression link between
ACE2 and Dopa Decarboxylase (DDC), which may explain that any
SARS‐CoV-2‐induced downregulation of ACE2 expression, previously
reported for SARS-CoV (Kuba et al., 2005), might disrupt the dopamine
and serotonin synthesis pathway, with the subsequent reduction of these
neurotransmitters in blood and brain as well as the serotonin-dependent
melatonin availability. This hypothesis should be corroborated and
correlated in COVID-19 patients affected by generalized anxiety disorder
and depressive symptoms (Huang & Zhao, 2020).
5. Melatonin supplementation against SARS-CoV-2 neurotropism: Dosage and
safety
To our knowledge, melatonin has not yet been tested in COVID-19
patients. Nevertheless, determining the safety and the precise doses of
this molecule to prescribe are challenges to be fulfilled. When orally
administered, melatonin has a low and variable bioavailability ranging
into 3-33 % (Andersen et al., 2016c; Di et al., 1997; Harpsoe et al.,
2015). Moreover, brain is the organ where melatonin reaches lower
concentration after its administration, which could justify the use of
higher doses to counteract the neuroinvasive potential of SARS-CoV-2
(Andersen et al., 2016b). In this regard, a new melatonin galenic
formulation with a higher bioavailability and faster absorption through
the CNS would be required, such as intravenous (IV) (Peschechera &
Veronesi, 2020) or intranasal administration (García-García et al.,
2016; Zetner et al., 2016).
Even though there are not sufficient short- or long-term studies with
exogenous melatonin focused on clarifying clinical safety (Seabra et
al., 2000), a significant number of studies indicate that the melatonin
is safe, even at doses 100 times higher than physiological
concentrations (Andersen et al., 2016a; Andersen et al., 2016b;
Brzezinski, 1997; Nickkholgh et al., 2011). Then, what are the
recommended melatonin dosing regimens in COVID-19 patients? The answer
would depend on the administration route and formulation. Reiter et al.
(Reiter et al., 2020a) have recently proposed a dose of 40 mg/day orally
to control the spread of the disease. In this context, we hypothesized
for COVID-19 patients in hyperinflammation phase, an oral dose of 100
mg/day or at least 1 mg/kg b.w would be required to establish whether
melatonin inhibits the neuro-impact of SARS-CoV-2, especially among
patients with severe neurological pathologies.
Melatonin co-administration with other drugs is also an attractive
strategy to improve the management of patients with COVID-19 and to
reduce the possible drug side effects. The potential of melatonin as an
adjuvant treatment (Zhang et al., 2020), as well as its combination with
mercaptopurine (Zhou et al., 2020) had been suggested as a feasible
therapy against SARS-CoV-2.
Since the effective vaccine and antiviral drugs are unavailable, it is
critically important to look for an alternative strategy for COVID-19
treatment. To this aim, melatonin is a serious candidate to consider
because it shows a low toxicity risk, the pharmacological efficacy
needed for the preventive treatment of COVID-19 infection and it has
several clinical actions from which COVID-19 patients could benefit. It
is time, therefore, to translate the therapeutic capacities of melatonin
for the improvement of clinical practice and protection of public health
in the current COVID-19 outbreak.
6. Concluding remarks
The current SARS-CoV-2 pandemic has stressed the public health systems
until unprecedented limits. The socio-economic consequences are expected
to be terrible worldwide and, most important, a health threat has spread
across the planet. Faced with this panorama, people are reacting with
panic to SARS-CoV-2 infection since several issues need solving.i ) The mechanisms associated with the infectiousness of
SARS-CoV-2 is not clear and their elucidation is urgently neededii ) SARS-CoV-2 causes different symptoms in different people and
a biological explanation is lacing. iii ) Clinicians are still
unsure whether people infected with COVID-19 can be reinfected;iv ) The lack of effective vaccines and specific antiviral drugs
targeted at SARS-CoV-2 which makes it difficult for treating or
controlling the pandemic. And v ) what will the long-term health
consequences of affected COVID-19 patients be? Given the panorama of
such an uncertain future facing us at the present time, we emphasize
that further investigation in the treatment with melatonin is urgently
required, as well as that clinical trials are strongly needed for the
best understanding of the impact of its administration in patients
affected by COVID-19.
Author Contributions: Conceptualization, A.R.; Writing Original
Draft Preparation, A.R. Writing-Review & Editing, A.R., E.R., E.G-M.,
F.L-M., G.E and R.J.R.; Supervision, A.R. and R.J.R; Funding
Acquisition, A.R.