Tunneling nanotubes may be a potential route for SARS-CoV-2 to spread to the nervous system, according to study findings published in Science Advances.
Patients who present with COVID-19 exhibit wide ranging symptoms from respiratory distress to problems with the gastrointestinal tract, cardiovascular system, and neurological manifestations. Although there is clear neural involvement for some patients, understanding how the virus enters the central nervous system (CNS) remains a mystery. In this study, researchers at the Institute Pasteur in France used cell lines to evaluate potential viral spread to the CNS.
Both human and murine neuronal cell lines did not appear to produce virus 3 days after infection. This was in contrast to control epithelial cell lines which had indications of viral replication following infection.
To evaluate whether the virus is exploiting intercellular communication, neuronal cells were cocultured with infected cells. At 24 hours, 36.4% of neuronal cells had anti-N antibody immunostaining which increased to 62.5% after 48 hours. Similarly, anti-S antibody immunostaining was observed in 21.8% of cells at 24 hours and 42.4% at 48 hours. These proteins were partially colocalized at 24 hours (r, 0.716) but not at 48 hours.
Immunostaining with anti-dsRNA indicated that after viral invasion, the virus could replicate in neuronal cells. To confirm this, staining with nonstructural protein 3 (nsp3) which is required for viral replication, found cytoplasmic puncta were positive for nsp3 after 48 hours.
Viral production was able to be halted after treatment with 30-40 mM remdesivir, in which 15.3% and 17.2% of cells had anti-S immunofluorescence after 24 and 48 hours of coculture of neuronal cells with SARS-CoV-2 infected cells, respectively.
To evaluate potential mechanisms for viral entry, the researchers assessed whether tunneling nanotubes could mediate the spread. Tunneling nanotubes were immunostained and anti-nsp3, anti-S, and anti-N antibodies were found in the tunneling nanotubes which were located between permissive and nonpermissive neuronal cells.
After infection, tunneling nanotubes were longer than before infection (mean, 34.48 vs 7.98 mm) and the number of tunneling nanotubes connecting the permissive and nonpermissive cells increased from 35% to 61.75% after infection.
This study was limited by the constraints of any in vitro study and should be confirmed in vivo.
“In conclusion, here, we show that SARS-CoV-2 is able to hijack [tunneling nanotubes] to spread between connected cells, indicating that this intercellular route could contribute to the pathogenesis of COVID-19 and the spreading of the virus to nonpermissive neuronal cell,” the researchers stated.
They also acknowledged that “These results pave the way to further investigations of the role of cell-to-cell communication in SARS-CoV-2 spreading to the brain in more physiological contexts (eg, the potential role of [tunneling nanotubes] in the spreading of the virus from the olfactory epithelium of the nasal cavity to the olfactory sensory neurons in the CNS and in contributing to the occurrence of the long COVID syndrome).”
Pepe Am, Pietropaoli S, Vos M, Barba-Spaeth G, Zurzolo C. Tunneling nanotubes provide a route for SARS-CoV-2 spreading. Sci Adv. Published online July 20, 2022. doi:10.1126/sciadv.abo0171