This is so interesting:
https://www.cell.com/cell-reports/fulltext/S2211-1247(21)01802-7
Synaptically connected neurons send proteins to each other packaged in exosomes. Previous studies showed protein transfer but I think this is the first large-scale screen, identifying 200 proteins (likely an underestimate because they only looked at ones passing through axons).
More evidence that our conventional models of neural signaling are radically incomplete!
...Not just protein-containing exosomes.
Neurotropic viruses also use extracellular vesicles to move directly from cell to cell.
This, combined with the well-documented retrograde axonal transport, is how neurotropic viruses are thought to directly infect the brain.
They start by infecting peripheral nerve endings and then, over a period of a few days, travel along nerves to the central nervous system, conveniently bypassing both blood-brain-barrier and the cerebrospinal fluid!
An example specifically related to coronaviruses:
"Neurotropic virus tracing suggests a membranous-coating-mediated mechanism for transsynaptic communication"
https://doi.org/10.1002/cne.23171
And an example specifically related to flaviviruses:
"Exosomes serve as novel modes of tick-borne flavivirus transmission from arthropod to human cells and facilitates dissemination of viral RNA and proteins to the vertebrate neuronal cells"
https://doi.org/10.1371/journal.ppat.1006764
Sam Gershman
Eric Pyrrhus