A recent article published in the Science Advances journal depicted that tunneling nanotubes (TNTs) offer a pathway for severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) neuronal spreading.
Although Coronavirus disease 2019 (COVID-19) patients typically exhibit severe respiratory symptoms and SARS-CoV-2 primarily affects the respiratory system, it also impacts other organs such as the liver, intestine, kidneys, brain, and heart. There have also been reports of varying gravity's neurological manifestations during COVID-19. Moreover, the neurological effects of SARS-CoV-2 infection are a significant problem in long COVID.
Given that various kinds of Coronaviruses (CoVs) have been documented to invade and survive in the central nervous system (CNS) (for example, Middle East respiratory syndrome CoV [MERS-CoV] and SARS-CoV), the capacity of SARS-CoV-2 to penetrate the CNS is anticipated. In addition, case studies have demonstrated that SARS-CoV-2 ribonucleic acid (RNA) was present in the brain tissue of individuals who passed away after having COVID-19.
Yet, it is unclear how SARS-CoV-2 enters the brain or how infection results in neurological symptoms since the main route for viral entry by endocytosis, the angiotensin-converting enzyme 2 (ACE2) receptors, are hardly detectable in the brain.
About the study
In the current research, the scientists examined the neuroinvasive capability of SARS-CoV-2. They determined if TNTs may play a role in the intercellular transmission of the virus.
TNTs are thin, actin-rich membrane channels that enable the direct transit of elements like viral particles, organelles, and amyloid proteins across different cells. Numerous viruses, like the herpes simplex virus and influenza virus, may communicate directly with naive cells by using TNTs to transfer their genomes, preventing drug targeting, and bypassing the host protection.
The team proposed that SARS-CoV-2 might use TNTs to propagate from permissive cells towards less permissive cells lacking the membrane receptor for virus entrance to spread viral pathogenicity and evade immune monitoring. Additionally, since the Vero E6 cell line has been utilized extensively for SARS-CoV-2 propagation, isolation, and antiviral assessment, they used it as an epithelial model to test this theory.
The SH-SY5Y cell line, a human cell type frequently utilized as a neuronal model and whose TNTs have also been extensively defined and are recognizable with considerable accuracy, was chosen by the researchers as a neuronal paradigm of nonpermissive cells. Primary neurons were preferred, but it was substantially challenging to distinguish between TNT-like structures in them. Moreover, it was also difficult to use the state-of-the-art cryo-electron tomography (cryo-ET) and cryo-correlative light and electron microscopy (cryo-CLEM) technologies that the team developed while using primary neuronal models.
The authors noted that the SH-SY5Y human neuronal cells were nonpermissive to SARS-CoV-2 via exocytosis or endocytosis-reliant pathways. However, they showed that it can be SARS-CoV-2-infected via a TNT-induced route when cocultured with Vero E6-permissive epithelial cells priorly infected with SARS-CoV-2 by employing confocal microscopy and establishing cellulo cryo-ET and cryo-CLEM. The team found that SARS-CoV-2 creates TNTs and subsequently leverages them to propagate to uninfected cells.
Cryo-ET and cellulo correlative fluorescence demonstrate that TNTs among permissive cells were connected with SARS-CoV-2. The results indicated that TNTs across permissive and nonpermissive cells contain viral replication sites and numerous vesicular structures, including double-membrane vesicles.
In addition, the data obtained by employing remdesivir and immunostaining for the viral replicative markers nonstructural protein 3 (nsp3) and J2 depicted that SARS-CoV-2 can multiply once within neural cells. The investigators further showed that SARS-CoV-2 could propagate between permissive cells via a secretion-autonomous channel by suppressing the ACE2-facilitated entrance of the virus using a neutralizing antibody. They postulate that TNTs hasten the spread of the infection, even among permissive cells.
Overall, the study data point to a hitherto unidentified mode of SARS-CoV-2 transmission, one that is probably employed to enter nonpermissive cells and intensify infection in permissive ones. The current findings shed light on the molecular basis of SARS-CoV-2 infection and transmission and the structure of the viral particles linked to TNTs.
Notably, the study findings suggest the function of TNTs in viral transmission, possibly increasing the effectiveness of viral spread across the body, within the constraints of an in vitro investigation. The available studies are centered primarily on blocking SARS-CoV-2 spike (S)-receptor interactions. The present study, in conjunction with existing reports, paves the way for additional investigations into the involvement of cell-to-cell interaction in SARS-CoV-2 propagation to the brain in more physiological settings and on substitute therapeutic strategies to impair viral diffusion.
In summary, the present work illustrated that SARS-CoV-2 could exploit TNTs to propagate between linked cells, suggesting that the intercellular route may play a role in COVID-19 pathogenesis and the dissemination of the virus to cells that are not susceptible to it, such as neuronal cells.