Coronaviruses (CoV) have garnered significant attention in recent years due to their ability to cause diseases in both animals and humans. These viruses, which belong to the family Coronaviridae and order Nidovirales, are enveloped, positive-sense single-stranded RNA viruses with a unique genetic makeup.
CoVs are responsible for a wide range of illnesses, including respiratory, enteric, hepatic, and neurological diseases, with varying degrees of severity in their hosts. This article delves into the diverse world of coronaviruses, focusing on their classification, human infections, and the potential for treatment.
Classification of Coronaviruses
Coronaviruses exhibit an intriguing genomic complexity, with the largest known RNA genomes, typically ranging from 27 to 32 kilobases (kb) in length. Their genomes contain multiple open reading frames (ORFs) arranged in a consistent order. The primary gene order includes a large replicase-transcriptase gene followed by structural genes (S-E-M-N) and accessory genes. This genetic arrangement is a hallmark of coronaviruses and distinguishes them from other viral families.
Based on their phylogenetic relationships and genomic structures, CoVs are subdivided into four genera: Alpha-, Beta-, Gamma-, and Delta-coronaviruses. Alpha- and Beta-CoVs primarily infect mammals, while Gamma- and Delta-CoVs have a natural reservoir in birds, occasionally infecting mammals.
However, in the early 2000s, two more HCoVs, HCoV-NL63 and HCoV-HKU1, were identified, expanding our understanding of these viruses.
Historical Perspective: Mild Infections to Lethal Outbreaks
Initially, human coronaviruses were believed to cause only mild illnesses, akin to the common cold. This perception changed dramatically in 2002 when the world was confronted with the Severe Acute Respiratory Syndrome (SARS) epidemic. SARS-CoV, a zoonotic virus, led to a mortality rate of over 10%, highlighting the potential severity of CoV infections in humans. In 2012, a similar scenario unfolded with the Middle East Respiratory Syndrome (MERS) outbreak, caused by MERS-CoV, which exhibited an even higher mortality rate, exceeding 35%.
Then, at the close of 2019, a new coronavirus, SARS-CoV-2, emerged in Wuhan, China. This virus, phylogenetically related to SARS-CoV, posed a far more significant threat to global public health due to its efficient transmission. By July 23rd, 2023, it had led to over 768 million confirmed cases and more than 6.9 million deaths worldwide. The disease associated with SARS-CoV-2, known as COVID-19, exhibits a wide range of clinical features, from asymptomatic cases to severe respiratory symptoms, pneumonia, acute respiratory distress syndrome (ARDS), multi-organ dysfunction, and death.
Seasonal Human Coronaviruses
Apart from the notorious SARS-CoV, MERS-CoV, and SARS-CoV-2, four seasonal human coronaviruses (sHCoVs) have been identified: HCoV-OC43, HCoV-229E, HCoV-NL63, and HCoV-HKU1. These viruses are widely distributed globally and are estimated to contribute to 15–30% of common cold cases in humans.
Although sHCoV infections are typically self-limiting, they can cause severe lower respiratory infections, especially in vulnerable populations such as infants, the elderly, and immunocompromised individuals. Additionally, sHCoVs have been implicated in enteric and neurological diseases, and there is a suggestion of HCoV-229E’s involvement in Kawasaki disease.
Notably, these four sHCoVs belong to two distinct taxonomic genera within the Coronaviridae family, Alpha and Beta. They also employ different receptors, which play a crucial role in determining tissue tropism and host range. HCoV-229E and HCoV-NL63 utilize cell surface enzymes as receptors, including aminopeptidase N (APN) for HCoV-229E and angiotensin-converting enzyme 2 (ACE2) for HCoV-NL63. In contrast, HCoV-OC43 and HCoV-HKU1 rely on 9-O-acetylated sialic acid as their receptor.
The Role of Spike Glycoprotein
The entry of sHCoVs into host cells is initiated by the spike (S) glycoprotein, which is anchored in the viral envelope. The spike protein is a trimeric class-I fusion glycoprotein, with each monomer weighing between 150 to 200 kDa following glycosylation. Initially, the spike is synthesized as an inactive precursor, which then assembles into an inactive homotrimer.
Cellular proteases cleave this homotrimer, generating two functional subunits: S1, containing the receptor-binding domain responsible for recognizing and attaching to the host receptor, and S2, the membrane-anchored subunit that houses the fusion machinery. The nascent spike protein is glycosylated in the endoplasmic reticulum (ER) during its synthesis, and those that pass quality control mechanisms are transported to the ER/Golgi intermediate compartment (ERGIC), where viral budding is presumed to occur.
Treatment Options for sHCoV Infections
Despite the potential for severe and life-threatening diseases caused by seasonal human coronaviruses, specific treatments for these infections remain elusive. The lack of effective therapeutics has prompted extensive research into potential antiviral agents.
One promising candidate is nitazoxanide, originally developed as an antiprotozoal agent and later approved for the treatment of infectious gastroenteritis. Nitazoxanide, along with second-generation thiazolides, has emerged as a new class of broad-spectrum antiviral drugs.
Recent investigations have explored the antiviral activity of nitazoxanide against three sHCoVs: HCoV-229E, HCoV-NL63, and HCoV-OC43. The results suggest that nitazoxanide exhibits potent inhibitory effects on HCoV replication, acting at a post-entry level and interfering with the maturation of spike glycoproteins in both Alpha- and Beta-sHCoVs.
Coronaviridae: An Evolving Virus Family
The Coronaviridae family stands out as one of the most rapidly evolving virus families due to its high genomic nucleotide substitution rates and recombination. This rapid evolution has led to the emergence of various coronaviruses with varying degrees of pathogenicity and transmission rates.
As of now, seven human coronaviruses have been identified, including HCoV-229E, HCoV-NL63, HCoV-OC43, and HCoV-HKU1, which circulate globally in the human population. Additionally, there are the highly pathogenic coronaviruses, namely SARS-CoV, MERS-CoV, and SARS-CoV-2, which have caused devastating pandemics, with SARS-CoV-2 leading to the unprecedented COVID-19 pandemic.
The COVID-19 pandemic has prompted significant efforts towards global vaccination. However, the emergence of SARS-CoV-2 spike variants that enhance virus spread and may affect vaccine efficacy, coupled with the short-lived protective immunity seen with HCoVs, underscores the importance of identifying antiviral drugs to mitigate the morbidity and mortality associated with coronaviruses.
Currently, two RNA-dependent RNA polymerase (RdRp) inhibitors, remdesivir and molnupiravir, as well as a viral protease inhibitor, Paxlovid, have been approved for use against SARS-CoV-2. However, there are no specific antiviral drugs or vaccines available for seasonal coronavirus infections.
Nitazoxanide: A Broad-Spectrum Antiviral
Nitazoxanide has garnered attention for its broad-spectrum antiviral properties. It, along with its active metabolite tizoxanide and second-generation thiazolides, has demonstrated effectiveness against various RNA pathogens, including rotaviruses, hepatitis C, and influenza and parainfluenza viruses, both in laboratory settings and clinical studies.
In the case of coronaviruses, the antiviral potential of nitazoxanide was first recognized in a canine strain of the virus (CCoV S-378) in canine A72 cells. Further studies showed that nitazoxanide was among the most effective compounds tested against various coronavirus strains. It exhibited significant efficacy against MERS-CoV as well. In the context of SARS-CoV-2, nitazoxanide was found to inhibit viral replication in different cell types, including human lung-derived Calu-3 cells, and in animal models.
Clinical Benefits of Nitazoxanide in COVID-19 Patients
Several clinical studies have reported the antiviral activity and clinical benefits of nitazoxanide in COVID-19 patients. However, it’s worth noting that not all studies have yielded positive results, highlighting the complexity of using antiviral drugs against SARS-CoV-2.
A recent study suggested that the efficacy of nitazoxanide against SARS-CoV-2 could depend on the dosage used and proposed the optimization of the drug’s formulation to improve its clinical efficacy.
Mechanism of Action: Spike Protein Maturation
The antiviral activity of nitazoxanide against coronaviruses is associated with its interference with spike (S) glycoprotein maturation. The spike protein plays a crucial role in coronavirus assembly, and hampering its maturation may hinder the formation of progeny virus particles.
Nitazoxanide was found to affect spike protein maturation at an Endo-H-sensitive stage, preventing its final processing. This effect is associated with the drug-mediated inhibition of ERp57, an endoplasmic reticulum-resident glycoprotein-specific thiol-oxidoreductase essential for the correct disulfide-bond architecture of certain viral proteins.
Multiple Mechanisms of Action
In addition to spike protein maturation, nitazoxanide’s antiviral activity may involve other mechanisms. These include:
- Interference with host cell energy metabolism: Nitazoxanide may decrease cellular ATP levels by mildly uncoupling mitochondrial oxidative phosphorylation, potentially impacting virus replication.
- Induction of autophagy: The drug has been shown to induce autophagy by inhibiting the Akt/mTOR/ULK1 signaling pathway, affecting virus replication.
- Activation of PKR: Nitazoxanide can activate protein kinase R (PKR), leading to the phosphorylation of eIF2-α, which is part of the host’s antiviral defense.
- Amplification of the host innate immune response: The drug may enhance innate immune responses by increasing RIG-I-like receptor activation, mitochondrial antiviral signaling protein activity, and interferon regulatory factor 3 activities.
Broad-Spectrum Antiviral Potential Against Seasonal HCoVs
In this study, nitazoxanide demonstrated potent antiviral activity against three seasonal human coronaviruses: HCoV-229E, HCoV-NL63, and HCoV-OC43. Importantly, the efficacy of nitazoxanide against these coronaviruses was comparable to the direct-acting antiviral remdesivir. Unlike remdesivir, which may develop resistance, nitazoxanide is less likely to lead to resistance due to its host-directed antiviral mechanism.
Moreover, nitazoxanide offers advantages such as oral administration, a well-established safety profile, and the potential for combination therapy with other drugs. While the study didn’t investigate HCoV-HKU1 due to its limited ability to grow in cell culture, the findings suggest that nitazoxanide could be a valuable tool in the treatment of seasonal coronavirus infections.
Coronaviruses are a diverse family of viruses with the potential to cause a wide range of diseases in animals and humans. While some human coronaviruses were originally believed to cause only mild illnesses, recent outbreaks of SARS, MERS, and COVID-19 have highlighted the severe consequences that these viruses can have on public health. Additionally, seasonal human coronaviruses continue to circulate and cause respiratory and other illnesses, especially in vulnerable populations.
Understanding the genetic makeup and mechanisms of these viruses, as well as identifying potential treatments like nitazoxanide, is crucial for mitigating
reference link : https://www.frontiersin.org/articles/10.3389/fmicb.2023.1206951/full