When it comes to spreading from cell to cell, viruses are the masters. To disseminate their progeny, many viruses encode glycoproteins that fuse virions to plasma membranes. According to a paper in the October 19 Nature Communications, those glycoproteins may unwittingly facilitate the intercellular transmission of their host’s cellular contents, including misfolded, prion-like proteins. Researchers led by Ina Vorberg, of the German Center for Neurodegenerative Diseases in Bonn, reported that two viral glycoproteins—VSV-G and the SARS-CoV2 S1 spike—dramatically boost the transfer of prions and misfolded tau between cells in culture. The viral ligands promote the release of extracellular vesicles, including any carrying proteopathic seeds, and their fusion to nearby cells. Once inside the recipient cell, the misfolded proteins co-opt their properly folded counterparts into aggregates.
- Viral capsid proteins promote release of extracellular vesicles and their fusion with neighboring cells.
- These glycoproteins include VSV-G and the SARS-CoV2 spike protein
- Both rev up cell-cell transmission of prions and misfolded tau.
“These results suggest a conceivable means whereby infections of various types might augment the probability of developing a neurodegenerative disease later in life,” commented Lary Walker of Emory University in Atlanta. “They also might help to explain why some infectious agents are stronger risk factors for Alzheimer's disease than are others.”
Prions seed aggregation by way of templated misfolding. Other proteins that cause neurodegeneration, including Aβ, tau, and α-synuclein, can propagate through the brain in a prion-like fashion. But how do the misfolded seeds spread from one cell to another? Numerous mechanisms have been proposed, ranging from the spewing of aggregates into the extracellular space, to tiny transcellular conduits, aka tunneling nanotubes, to gap junctions (Feb 2012 news; Tardivel et al., 2016; Sep 2021 news). Extracellular vesicles, including exosomes, are another potential vector. These tiny membrane packages ferry cellular material, including misfolded proteins Aβ and tau, between cells (Oct 2015 news; Dec 2020 news). Notably, viral infection instigates the formation and release of extracellular vesicles containing viral protein, and viral glycoproteins facilitate membrane fusion.
Might viral glycoproteins amplify proteopathic transfer via exosomes then? First author Shu Liu and colleagues addressed this with a series of cell culture models. They started by pairing a characterized prion—the NM prion domain from the yeast Sup35 protein—with the vesicular stomatitis virus glycoprotein. Made by rhabdoviruses, which include rabies, VSV-Gs strengthen the docking of EVs with cells by latching onto LDL receptors. Sup35-NM prions are known to spread between cells via direct contact as well as via EVs. Would adding VSV-G enhance prion dissemination?
Indeed, the scientists found that the glycoprotein ramped up prion transmission. HEK-293 or N2a neuroblastoma cells harboring aggregates of Sup35-NM poorly induced aggregation in co-cultured recipient cells containing only the soluble form. However, when the donor cells were transfected with VSV-G, aggregation in recipient cells ramped up dramatically. The researchers ultimately tied this enhanced transmission to both an increase in EV release from donor cells and more efficient fusion of EVs to recipient cells. VSV-G also cranked up prion transmission in primary human astrocytes.
This viral glycoprotein also helped transmit PrPSC, the misfolded form of cellular prion protein, between cells. Unlike the intracellular Sup35 protein, PrPc is membrane-bound, thus extending the viral glycoprotein’s sphere of influence.
How about tau? To investigate, the researchers used HEK293 cell lines that express the repeat domains of tau—complete with pathogenic mutations—that are highly prone to aggregation. They induced aggregation in these cells by exposing them to homogenates extracted from postmortem brain samples of people who had died with different tauopathies, including AD, cortical basal degeneration, progressive supranuclear palsy, or frontotemporal lobar degeneration. They then co-cultured these aggregate-bearing cells with recipient cells expressing fluorescently labeled tau. Remarkably, recipient cells only lit up with tau aggregates when co-cultured with donor cells expressing VSV-G. This suggested that the glycoprotein was crucial for transfer of misfolded tau between cells.
Finally, the researchers asked whether another viral glycoprotein—the S1 spike protein from the infamous SARS-CoV2—would also amp up the intercellular transfer of protein miscreants. Studding the coronavirus capsid, the spike facilitates the entry of the virus into cells by binding to ACE2 receptors on their surface. The researchers transfected donor HEK cells with S1 and co-cultured them with HEK cells expressing ACE2, or Vero monkey kidney cells that are highly susceptible to coronavirus infection. They found S1 behaved similarly to VSV-G. Although the spike protein did not increase the release of EVs from donor cells, it enhanced the transmission of Sup35-NM as well as misfolded tau, leading to aggregation in recipient cells. S1-bearing EVs isolated from donor cell cultures could directly induce misfolding in recipient cells.
The findings suggest a mechanism through which viral infections might egg on the spread of protein pathology in the brain. “Although the study is limited to overexpression systems using immortalized cell lines, the results are striking and may have translational relevance to virus infection-mediated enhancement of tau propagation via EVs, including by COVID-19, HSV, or HIV infection,” wrote Tsuneya Ikezu of the Mayo Clinic in Jacksonville, Florida. However, as Ikezu and other commentators pointed out, in vivo experiments will be crucial to address if, and how, this occurs within the brain.
The idea that viruses instigate or exacerbate neurodegenerative diseases by encouraging release and transmission of toxic proteins is not far-fetched. Previous studies reported that retroviral glycoproteins boosted the infectivity of prions, and also triggered the formation of Aβ plaques (Leblanc et al., 2006; Kodidela et al., 2019). In regard to SARS-CoV2, studies have found that not only can the virus infect the brain, but the spike protein itself can go rogue and cross the blood-brain barrier (Heneka et al., 2020; Jan 2021 news; Rhea et al., 2020).
Herpesviruses, such as HSV-1, also infect the CNS, and have been implicated in AD. Vorberg noted that, like many viruses, HSV-1 encodes its own fusion protein—glycoprotein B—which forms a complex with other viral proteins to fuse membranes together. “Importantly, herpesvirus glycoproteins and HIV envelope proteins are also sorted onto EV,” she wrote to Alzforum.
Vorberg’s group is analyzing patient brain samples to get an idea of which viruses, and which types of protein aggregates, might sway neurodegenerative disease. They also plan to study mouse models. “Our work unveils an important aspect of proteopathic seed transmission. Uptake, and especially release, of proteopathic seeds in the cytosol are bottlenecks that might represent targets for drug development,” she added.—Jessica Shugart
- Mice Tell Tale of Tau Transmission, Alzheimer’s Progression
- Microglia Share Synuclein Aggregates with Each Other Via Nanotubes
- Deadly Delivery: Microglia May Traffic Tau Via Exosomes
- Fatal Gift Wrap: Neurons Fall for Packaged Tau Oligomers
- How Does COVID-19 Affect the Brain?
- Tardivel M, Bégard S, Bousset L, Dujardin S, Coens A, Melki R, Buée L, Colin M. Tunneling nanotube (TNT)-mediated neuron-to neuron transfer of pathological Tau protein assemblies. Acta Neuropathol Commun. 2016 Nov 4;4(1):117. PubMed.
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- Kodidela S, Gerth K, Haque S, Gong Y, Ismael S, Singh A, Tauheed I, Kumar S. Extracellular Vesicles: A Possible Link between HIV and Alzheimer's Disease-Like Pathology in HIV Subjects?. Cells. 2019 Aug 24;8(9) PubMed.
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- Rhea EM, Logsdon AF, Hansen KM, Williams LM, Reed MJ, Baumann KK, Holden SJ, Raber J, Banks WA, Erickson MA. The S1 protein of SARS-CoV-2 crosses the blood-brain barrier in mice. Nat Neurosci. 2020 Dec 16; PubMed.
No Available Further Reading
- Liu S, Hossinger A, Heumüller SE, Hornberger A, Buravlova O, Konstantoulea K, Müller SA, Paulsen L, Rousseau F, Schymkowitz J, Lichtenthaler SF, Neumann M, Denner P, Vorberg IM. Highly efficient intercellular spreading of protein misfolding mediated by viral ligand-receptor interactions. Nat Commun. 2021 Oct 19;12(1):5739. PubMed.