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Outbreaks of emerging infections, such as COVID-19 pandemic especially, confront health professionals with the unique challenge of treating patients. With no time to discover new drugs, repurposing of approved drugs or in clinical development is likely the only solution. Replication of coronaviruses (CoVs) occurs in a modified membranous compartment derived from the endoplasmic reticulum (ER), causes host cell ER stress and activates pathways to facilitate adaptation of the host cell machinery to viral needs. Accordingly, modulation of ER remodeling and ER stress response might be pivotal in elucidating CoV-host interactions and provide a rationale for new therapeutic, host-based antiviral approaches. The sigma-1 receptor (Sig-1R) is a ligand-operated, ER membrane-bound chaperone that acts as an upstream modulator of ER stress and thus a candidate host protein for host-based repurposing approaches to treat COVID-19 patients. Sig-1R ligands are frequently identified in in vitro drug repurposing screens aiming to identify antiviral compounds against CoVs, including severe acute respiratory syndrome CoV-2 (SARS-CoV-2). Sig-1R regulates key mechanisms of the adaptive host cell stress response and takes part in early steps of viral replication. It is enriched in lipid rafts and detergent-resistant ER membranes, where it colocalizes with viral replicase proteins. Indeed, the non-structural SARS-CoV-2 protein Nsp6 interacts with Sig-1R. The activity of Sig-1R ligands against COVID-19 remains to be specifically assessed in clinical trials. This review provides a rationale for targeting Sig-1R as a host-based drug repurposing approach to treat COVID-19 patients. Evidence gained using Sig-1R ligands in unbiased
The newly emerged 2019 novel coronavirus (CoV), named as severe acute respiratory syndrome CoV-2 (SARS-CoV-2), has been associated with high infection rates and has spread rapidly to become a pandemic (COVID-19 pandemic) since its identification in patients with severe pneumonia in Wuhan, China. Unfortunately, no vaccine has yet been approved to treat human CoVs and the discovery and development of new drugs will require years. Accordingly, repurposing of approved drugs or drugs in clinical development has emerged as a feasible approach to reduce the time compared to
CoV replication is structurally and functionally associated with the endoplasmic reticulum (ER) (
The sigma-1 receptor (Sig-1R) acts as an upstream modulator of ER stress. Sig-1R is a ligand-operated, membrane-bound chaperone that normally reside at the ER-mitochondrion contact called the (mitochondrion-associated ER membrane (MAM), where it regulates ER-mitochondrion signaling and ER-nucleus crosstalk (
As it regards to its potential antiviral activity, Sig-1R ligands are frequently identified in in vitro drug repurposing screens aiming to identify antiviral compounds against SARS-CoV-2 and other CoVs. Mechanistically, Sig-1R is involved in cellular stress pathways which are used by viruses to promote viral replication (
Here pharmacological and genetic data supporting a role for Sig-1R in viral infection are collected and summarized, with a focus on CoV in general and SARS-CoV-2 in particular. Targeting Sig-1R is identified as a potential drug repurposing approach to treat COVID-19 patients that, unlike virus-targeted antiviral agents, addresses adaptive cellular mechanisms of host cells that are crucial for viral infection.
The first insight about a potential role for Sig-1R ligands as antivirals was probably published in 1984 ( Drug class.
aIC50. CHIKV, Chikungunya virus; HCV, Hepatitis C virus; FLUAV, Influenza A virus; H5N1, Avian influenza A H5N1 virus, other subtypes; CCHFV, Crimean-Congo hemorrhagic virus; HSV-1, Herpes simplex virus type 1; JCV, JC (John Cunningham) virus; DENV, Dengue virus; HCoV-229E, Human coronavirus strain 229E; MARV, Marburg hemorrhagic fever virus; MERS-CoV, East respiratory syndrome coronavirus; SARS-CoV, Human coronavirus strain OC43 (HCoV-OC43) Severe acute respiratory syndrome coronavirus; EBOV, Ebola virus; HIV-1, Human immunodeficiency virus type 1; SARS-CoV-2, Severe acute respiratory syndrome coronavirus type 2; EBV, Epstein-Barr virus.
Intended therapeutic effect
Compound
Sigma-1 receptor affinity
Antiviral activity
Ki or IC50a (nM)
References
Virus
References
Antiarrhythmic
Amiodarone
1.4–2.1
EBOV
(
335a
(
(
(
HCV
(
(
(
SARS-CoV
(
SARS-CoV-2
(
Antidepressant Anxiolytic
Amitriptyline
287
FLUAV (H5N1)
(
216a
HCV
(
300a
Antimalarial
Amodiaquine
355a
EBOV
(
DENV
(
MARV
(
MERS-CoV
(
SARS-CoV
(
SARS-CoV-2
(
Antihistaminic
Astemizole
43a
EBOV
(
MERS-CoV
(
SARS-CoV
(
Antiallergic
Azelastine
274a
HCV
(
Antitussive
Benproperine
19a
HCV
(
Antiparkinsonian Treatment dystonia and extrapyramidal side effects of antipsychotics
Benztropine
65a
EBOV
(
(
HCV
(
(
MERS-CoV
(
SARS-CoV SARS-CoV-2
(
Antiarrhythmic and antianginal
Bepridil
365a
EBOV
(
Antihypertensive
Carvedilol
1570a
HCV
(
Serotonergic
CGS 12066B
1180a
HCV
(
Antimalarial
Chloroquine
108.6
CCHFV
(
2300a
CHIKV
(
(
DENV
(
EBOV
(
(
FLUAV (H1N1 and H3N2)
(
FLUAV (H5N1)
(
HCoV-229E
(
HCoV-OC43
(
HIV-1
(
MARV
(
MERS-CoV
(
(
SARS-CoV
(
(
(
SARS-CoV-2
(
(
Antihistaminic Antiparkinsonian
Chlorphenoxamine
1760a
MERS-CoV
(
SARS-CoV
(
Antipsychotic
Chlorpromazine
146
CCHFV
(
200a
CHIKV
(
1070a
EBV
(
FLUAV
(
HCoV-229E
(
HCV
(
MERS-CoV
(
(
SARS-CoVSARS-CoV-2
(
(
Antihistaminic Antivertigo
Cinnarizine
22
HCV
(
119a
Antihistaminic
Clemastine
67
EBOV
(
505a
SARS-CoV-2
(
Neuroprotectant
Clobenpropit
1080a
HCV
(
Estrogen receptor modulator
Clomiphene
4.7–12
EBOV
(
Ovulation stimulator
195a
(
HCV
(
(
Antidepressant
Clomipramine
195a
EBOV
(
HCV
(
MERS-CoV
(
SARS-CoV SARS-CoV-2
(
Antihistaminic Antitussive
Cloperastine
20
SARS-CoV-2
(
277a
Antifungal
Cycloheximide
1030a
MERS-CoV
(
SARS-CoV
(
Antiallergic Antihistaminic
Cyproheptadine
284a
HCV
(
930
(
Antidepressant
Desipramine
1190a
HCV
(
1987
Antiallergic
Desloratadine
1510a
HCV
(
Antidepressant
Doxepin
394a
CHIKV
(
HCV
(
Selective sigma-1 receptor antagonist
E-52862 (S1RA)
17
SARS-CoV-2
(
Antimigraine
Flunarizine
28a
HCV
(
Antidepressant
Fluoxetine
240
HCV
(
949a
HCV
Antipsychotic
Flupentixol
70a
EBOV
(
Antipsychotic
Fluphenazine
13
EBOV
(
62
HCV
(
109a
MERS-CoV
(
(
SARS-CoV SARS-CoV-2
(
Antipsychotic
Fluspirilene
150
MERS-CoV
(
380a
SARS-CoV
(
563a
SARS-CoV-2
(
Antipsychotic
Haloperidol
0.2
EBV
(
1.1
HCV
(
1.1a
MERS-CoV
(
1.2
SARS-CoV
(
2.4
SARS-CoV-2
(
3
6.5–7.3
17a
73a
Antimalarial
Hydroxychloroquine
2120a
DENV
(
HIV-1
(
MERS-CoV
(
SARS-CoV
SARS-CoV-2
(
(
(
Antiallergic
Hydroxyzine
342a
HCV
(
Antihistaminic
46
Anxiolytic
Neuroprotectant Anticonvulsant Antihypertensive
Ifenprodil
1.02
FLUAV (H5N1)
(
2–2
HCV
(
5.5a
28–34
Antidepressant
Imipramine
343
HCV
(
529a
Antiadictive
Indatraline
737a
HCV
(
Antihistaminic
Ketotifen
3800a
HCV
Antidepressant
Lofepramine
2520
HCV
(
100% inh. at 10
Antimigraine
Lomerizine
37a
EBOV
(
Antidiarrheal
Loperamide
271
HCoV-229E
(
MERS-CoV
SARS-CoV
SARS-CoV-2
(
Antidepressant
Maprotiline
37a
EBOV
(
Antimalarial
Mefloquine
2560a
JCV
(
MERS-CoV
(
SARS-CoV
(
SARS-CoV-2
(
Antihistaminic
Methdilazine
167a
CHIKV
(
Sigma ligand
PB28
0.38
SARS-CoV-2
(
10–13
Sigma ligand
PD-144418
0.08
SARS-CoV-2
(
0.46
Antipsychotic Antiemetic Anxiolytic
Perphenazine
8
CHIKV
(
13
HCV
(
21a
(
45–53
104a
Antipsychotic Treatment of Tourette syndrome and resistant tics
Pimozide
139
EBOV
(
508
HCV
(
337a
Antipsychotic
Piperacetazine
823a
EBOV
(
Antipsychotic Antiemetic Anxiolytic
Prochlorperazine
232a
EBOV
(
(
HCV
(
(
Endogenous steroid Menopausal hormone therapy
Progesterone
173–196
SARS-CoV-2
(
1960a
260–338
Antiallergic Antihistaminic
Promethazine
857a
EBV
(
MERS-CoV
(
SARS-CoV SARS-CoV-2
(
Antidepressant
Protriptyline
307a
HCV
(
Antimalarial
Quinacrine
953a
EBOV
(
Antiarrhythmic
Quinidine
570
HCV
(
5480a
Serotonergic
Quipazine-6N
1250a
HCV
(
Estrogen receptor modulator Treatment of osteoporosis in postmenopausal women
Raloxifene
38
HCV
(
122a
(
(
Sigma ligand
Rimcazole
260a
HCV
(
500a
820
908
Antitumoral (breast cancer)
Ritanserin
190a
HCV
(
Antiasthmatic
Salmeterol
151a
HCV
(
Antidepressant Anxiolytic
Sertraline
57
EBOV
(
260a
Sigma ligand
Siramesine
17a
SARS-CoV-2
(
Estrogen receptor modulator
Tamoxifen
34–26
HCV
(
Antitumoral (breast cancer)
367a
HSV-1
(
(
MERS-CoV
(
SARS-CoV SARS-CoV-2
(
Antifungal
Terconazole
159a
EBOV
(
MERS-CoV
(
SARS-CoV SARS-CoV-2
(
Antiemetic
Thiethylperazine
528a
CHIKV
(
MERS-CoV
(
SARS-CoV SARS-CoV-2
(
Antipsychotic
Thioridazine
286a
CHIKV
(
EBOV
(
Antipsychotic
Thiothixene
353a
MERS-CoV
(
SARS-CoV
(
Estrogen receptor modulator
Toremifene
220a
EBOV
(
Antitumoral (breast cancer)
HCV
(
MERS-CoV
(
SARS-CoV SARS-CoV-2
(
Antipsychotic
Trifluoperazine
15–21
EBV
(
54
HCV
(
125a
(
345a
Antipsychotic Antiemetic
Triflupromazine
154
HCV
(
470a
MERS-CoV
(
605a
SARS-CoV
(
Antihypertensive
Verapamil
258a
FLUAV
(
Antiarrhythmic
SARS-CoV-2
(
Antipsychotic
(−)-Butaclamol
40
HCV
(
95.7
157
183a
Antipsychotic
(±)-Butaclamol
343a
HCV
(
Drugs binding Sig-1R showing antiviral activity were identified in three
In the third of these screening studies, 1280 compounds, many in clinical trials or approved for therapeutic use, were assayed for their ability to alleviate the HCV-induced cytopathic effect on the engineered cell line n4mBid (
Changes in cell death induced by avian influenza A (FLUAV) H5N1 virus in A549 lung epithelial cells were explored using RNA interference (RNAi) screening methods. These screens identified multiple genes for which knockdown altered cell viability and drugs targeting some of these genes were assayed for their potential antiviral activity. The neurological drug ifenprodil increased cell viability
Regarding filoviruses, a systematic
Finally, inhibition of the cytopathic effect induced by Chikungunya virus and other alphaviruses (Semliki Forest virus and Sindbis virus) was found for chlorpromazine, doxepin, methdilazine, perphenazine, thiethylperazine, thioridazine and chloroquine (
SARS-CoV-2 (severe acute respiratory syndrome-related CoV type 2), the causative virus of COVID-19 pandemic, belongs to the broad family of positive-sense single-stranded RNA (+ssRNA) CoV. Other CoV also cause illnesses ranging from common cold to more severe diseases such as Middle East respiratory syndrome (MERS). It is the seventh known CoV to infect people, after 229E, NL63, OC43, HKU1, MERS-CoV, and the original SARS-CoV (
In this section, data supporting the involvement of Sig-1R and therapeutic potential of Sig-1R ligands against CoV infection is summarized.
Chloroquine and hydroxychloroquine bind to Sig-1R (
Chloroquine and its hydroxy analog were by far the most popular drugs proposed initially for treatment and prophylaxis of COVID-19: 208 interventional clinical trials registered on the NIH site involve treatment with these drugs, alone or in combination (
The antiarrhythmic amiodarone, a non-selective but high affinity sigma-1 ligand, was reported to inhibit the spreading
A set of 348 FDA-approved drugs was screened in cell cultures infected with MERS-CoV (
In another study, a library of 290 compounds with FDA approval or in advanced clinical development was screened for antiviral activity against MERS-CoV and SARS-CoV (
In a recent repositioning study, 48 FDA-approved drugs, including 35 drugs pre-selected by their activity against SARS-CoV snd 13 drugs recommended from infectious diseases specialists, were assayed for their antiviral activity against SARS-CoV-2 in Vero cells (
In a recent paper, targeting Sig-1R was highlighted based on findings of a SARS-CoV-2 protein interaction map and pharmacological data (
Finally, in a recent publication, quantitative high-content morphological profiling coupled with an AI-based machine learning strategy was applied to identify efficacious single agents against SARS-CoV-2 (
Inhibition of viral entry has been reported for non-selective sigma-1 ligands in a number of studies (
Inhibition of HCV entry into Huh-7 human hepatoma cells by sigma-1 ligands was demonstrated in pharmacology studies (
Sig-1R normally resides at the ER, typically at the MAM, but when cells undergo stress (as expected following viral infection) the Sig-1R translocates to the peripheral ER network and plasma membrane to regulate a variety of cell surface proteins (
Proposed model of severe acute respiratory syndrome CoV-2 (SARS-CoV-2) entry by endocytosis: Potential role of Sigma-1 receptors (Sig-1Rs) via BiP interaction. Abbreviations and bibliographic references for the role of Sig-1R are provided in the text.
The endocytic pathway (receptor-dependent endocytosis) is a basic mechanism for entry of CoV, including SARS-CoV, MERS-CoV and SARS-CoV-2, into host cells (
Endosomes and maturation of endosomes into a lysosome is featured by their acidic internal pH, which is required for acid proteases and critical for SARS-CoV-2 processing and internalization (
Some drugs recognized as antiviral agents are lipophilic amines/weak bases that accumulate and preclude acidification of lysosomes, thus inhibiting virus internalization and post-internalization trafficking to the site of replication (
A variety of marketed drugs fit within general physicochemical properties of lysosomotropic agents. Essentially, drugs with a ClogP > 2 and pKa between 6.5 and 11 can accumulate into lysosomes (
In this section, evidence gained through gene silencing approaches are discussed. The antiviral activity exerted by numerous sigma-1 ligands in drug repurposing
Overall, data from Sig-1R deficient cells indicate that Sig-1R is a host cellular factor recruited for HCV infection, downstream entry, delivery and primary translation of viral RNA genome that regulates early stages of HCV RNA replication (
In this section, evidence gained from colocalization and interactome map studies are discussed. Sig-1R was found to colocalize with NS proteins of the HCV replication complex (
Recently, a SARS-CoV-2 protein interaction map reveled a physical interaction with Sig-1R (
Positive-strand RNA viruses, including HCV and SARS-CoV, sequester host cell ER membranes to assemble viral replication. A network of modified perinuclear rough ER that integrates convoluted membranes, interconnected double-membrane vesicles and vesicle packets has been described (
CoV infection of cultured cells causes ER stress and induces the unfolded protein response (UPR), the ER-specific stress response, and their downstream signals (
Proposed model of severe acute respiratory syndrome CoV-2 (SARS-CoV-2)-mediated unfolded protein response (UPR) signaling: Potential role of Sigma-1 receptors (Sig-1Rs) via interaction with master UPR regulators. Abbreviations and bibliographic references for the role of Sig-1R are provided in the text.
The burst of protein synthesis overloading the ER folding capacity, extensive rearrangement of the ER membrane during viral replication and viral proteins such as S (
What about Sig-1R? ER stress/UPR induces Sig-1R expression through the PERK/eIF2α/ATF4 pathway (ATF4 binds to the 5' flanking region of Sig-1R gene to upregulate its transcription) (
ER remodeling is a key early element of ER stress response induced by CoVs. As discussed before, CoVs benefit from endomembrane compartments and induce the growth and remodeling of host cell ER membranes to form a reticulovesicular network (
Viruses have evolved mechanisms to disturb host cell Ca2+ homeostasis and increase intracellular Ca2+ as Ca2+ is essential for virus entry, replication, maturation and release (
Proposed model of severe acute respiratory syndrome CoV-2 (SARS-CoV-2)-mediated disturbance of host cell calcium (Ca2+) homeostasis: Potential role of Sigma-1 receptors via interaction with Ca2+ channels in the plasma membrane and ER. CaM kinases, Ca2+/calmodulin-dependent protein kinases; DAG, diacylglycerol; IP3, inositol 1,4,5-triphosphate; IP3Rs, inositol 1,4,5-triphosphate receptor-gated channels; IP3R1 mediates ca2+ release from ER to cytosol and IP3R3 to mitochondria. They are activated not only by IP3, but also by low Ca2+ concentrations through a process often referred to as Ca2+-induced Ca2+ release (CICR). High cytosolic Ca2+ concentration can instead inhibit IP3Rs; PKC, protein kinase C; mNCE, mitocondral Na+/Ca2+ and 2H+/Ca2+ exchangers. They slowly eject accumulated Ca2+ back into the cytosol; mPTP, mitochondrial permeability transition pore. Once intramitochondrial Ca2+ rises above a certain threshold, this voltage- and Ca2+-dependent high-conductance channel in the inner membrane opens, activating cell death mechanisms; mUP, mitochondrial uniporter. It is gated by Ca2+ in a biphasic manner. Ca2+ uptake into mitochondria is facilitated by Ca2+/calmodulin, but sustained cytosolic Ca2+ levels inactivate the uniporter, preventing further Ca2+ uptake; RyR, ryanodine receptor. It is a Ca2+-gated Ca2+ channels (CICR); SERCA, sarco/endoplasmic reticulum Ca2+ ATPase. It restores ER with Ca2+. Bibliographic references for the role of Sig-1R are provided in the text.
Sig-1R binds in a dynamic, reversible and Ca2+-dependent manner to the ER lumenal chaperone and stress sensor BiP (
In addition to its interaction with its co-chaperone BiP, Sig-1R also chaperones the ER resident transmembrane protein IRE1 (
Finally, CoV infection (inclusive of SARS-CoV, MERS-CoV and the new SARS-CoV-2) has been demonstrated to induce autophagy (
The author confirms being the sole contributor of this work and has approved it for publication.
JV was a full-time employee in ESTEVE PHARMACEUTICAS at the time of review.