On 11th March 2020, the World Health Organization (WHO) announced the global spread of the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) as the coronavirus disease 2019 (COVID-19). 

Despite the discovery of effective COVID-19 vaccines and the subsequent commencement of vaccination programs globally, SARS-CoV-2 infections have been widely reported due to the emergence of new variants that can escape immune responses induced through both vaccination and natural infection. Thus, there remains a considerable need for novel, quick-deployable, and efficient antiviral therapies.

Study: SARS-CoV-2 infects human brain organoids causing cell death and loss of synapses that can be rescued by treatment with Sofosbuvir. Image Credit: Gorodenkoff / Shutterstock.com

Background

In addition to respiratory distress, COVID-19 patients also experience adverse direct or indirect impacts on the central nervous system (CNS). Several neurological symptoms, such as stroke, epilepsy, anosmia, ageusia, hallucinations, and encephalopathy, have been associated with SARS-CoV-2 infection. 

One mouse model revealed that the SARS-CoV-2 spike S1 protein can cross the blood-brain barrier, thereby indicating that the virus can infect the brain and induce neurological symptoms. Autopsy reports of patients who died from COVID-19 showed the presence of SARS-CoV-2 in cortical neurons. Additionally, the possibility of vertical transmission of SARS-CoV-2 to the fetus has been found, which can affect fetal brain development. 

Human brain organoids are three-dimensional models of the brain that mimic cellular and molecular aspects of human embryonic and fetal developmental stages. Previous studies revealed that human brain cortical functional organoids can closely recapitulate the early stages of neural development and organize cortical networks. 

About the study

In a recent PLoS Biology study, researchers discuss how SARS-CoV-2 infects cortical neurons and damages their synapses that form the connection between brain cells. This piece of research not only evaluates the risk of SARS-CoV-2 infection in human brain cells but also analyzes its impact on the developing human brain.

The TISSUES database helped identify proteins associated with SARS-CoV-2 infection in the human brain. Some of the proteins expressed in the brain include transmembrane serine protease 2 (TMPRSS2), angiotensin-converting enzyme 2 (ACE2), neuropilin-1 (NRP1), and CD147, but not CD26.

These entry factor proteins are expressed at a reduced level in the CNS as compared to other organs. ACE2 and TMPRSS2, for example, are less expressed compared to NRP1, which is highly expressed in the cerebral cortex and hippocampus. However, BSG/CD147 gene is highly expressed in all brain regions.  

To test whether SARS-CoV-2 could infect the developing human brain, researchers created eight-week-old human brain cortical organoids (BCO) using dermal fibroblasts from healthy donors. Organoids were infected with SARS-CoV-2 to determine whether BCOs were vulnerable to SARS-CoV-2 infection.

One of the important aspects of this research was to identify United States Food and Drug Administration(FDA)-approved antiviral drugs that can alleviate neurological symptoms caused by SARS-CoV-2 infection. In this study, BCO infected with the influenza A virus, using the same experimental design, was used as control.

Study findings

Sofosbuvir (SOF) is an antiviral drug that has received approval from the FDA for hepatitis C (HCV) treatment. Notably, this drug can also inhibit other single-stranded viruses, including coronaviruses. As a result, the current study assessed the efficacy of SOF in alleviating neurological manifestations in COVID-19 patients.

Mechanistically, SOF inhibits HCV replication by restricting the activity of the ribonucleic acid (RNA)-dependent RNA polymerase (RdRp). A high degree of sequence and structural similarity was found between the RdRp of SARS-CoV-2 and HCV.

Importantly, SOF-binding residues are conserved amongst several coronaviruses, including SARS-CoV-2. Considering these observations, the authors hypothesized that SOF could effectively inhibit SARS-CoV-2 replication.

A varied range of SOF dosages was used for BOC treatment. To this end, an increased dose of SOF was found to effectively decrease intracellular SARS-CoV-2 RNA levels.

Nevertheless, the highest inhibition of SARS-CoV-2 replication, without inducing cell death, occurred at 20 μM SOF concentration. Furthermore, the efficacy of SOF was validated by analyzing intracellular viral RNA and the number of viable viruses present in the supernatants of SARS-CoV-2-infected BCOs treated with SOF. 

Importantly, a reduced number of infectious viruses were detected after antiviral treatment. Immunoblotting and immunostaining experiments further validated the aforementioned findings.

Therefore, experimental findings underscored the efficacy of SOF in fighting COVID-19. Notably, SOF treatment not only reduced SARS-CoV-2 viral protein levels but also decreased virus-induced cell death.

Nestin+ NPCs and MAP2+ neurons were found to be susceptible to SARS-CoV-2 infection. An increased level of nucleocapsid protein of SARS-CoV-2 in BCO was linked with increased cell death in both neurons and neural progenitor cells (NPC).

To assess the impact of COVID-19 on synaptic integrity, the number of excitatory synapses in neurons was quantified using Synapsin 1, vGLUT1, and PSD95 antibodies. A significant decrease in pre-synaptic proteins was observed during SARS-CoV-2 infection, which was effectively alleviated using SOF treatment.

Conclusions

Although experimental findings demonstrated the effectiveness of SOF in improving the neurological conditions of COVID-19 patients, more clinical evaluations are needed for further validation. Nevertheless, SOF appears to be a promising drug for preventing the development of neurological symptoms in COVID-19 patients.

Written by

Dr. Priyom Bose

Priyom holds a Ph.D. in Plant Biology and Biotechnology from the University of Madras, India. She is an active researcher and an experienced science writer. Priyom has also co-authored several original research articles that have been published in reputed peer-reviewed journals. She is also an avid reader and an amateur photographer.