How rabies virus moves through nerve cells, and how it might be stopped

Cells in red against black background

To successfully infect its host, the rabies virus must move from the nerve ending to the nerve cell body where it can replicate. In a study published July 20 in the journal PLoS Pathogens, researchers from Princeton University reveal that the rabies virus moves differently compared to other neuron-invading viruses and that its journey can be blocked by a drug commonly used to treat amoebic dysentery.

Most viruses only infect the nervous system accidentally when the immune system is compromised. But some “neurotropic” viruses have evolved to target neurons as part of their normal infectious cycle. The rabies virus, for example, is transmitted when an infected animal bites into a host’s muscle. It then spreads into the end terminals of motor neurons innervating the muscle and travels along the neurons’ long axon fibers to the neuronal cell bodies. From there, the virus can spread throughout the central nervous system and into the salivary glands, where it can be readily transmitted to other hosts. Though rabies infections in humans are rare in the United States, the virus kills nearly 60,000 people annually.

Alpha herpesviruses, such as herpes simplex viruses, also enter peripheral nerve terminals and move along axons to the neuronal cell body, where they can lie dormant for the life of the host.

“Transport to the neuronal cell body is not a passive process, but an active one relying on the neuron’s own motor proteins and microtubule tracks,” said Lynn Enquist, Princeton’s Henry L. Hillman Professor in Molecular Biology, a professor of molecular biology and the Princeton Neuroscience Institute, and the study’s senior author. “Virus particles must engage this machinery for efficient transport in axons, otherwise infection cannot start.”

Viruses moving in a narrow nerve cell
Twenty-four hours after infecting nerve terminals, the rabies virus (red) reaches the cell bodies (green) of control neurons (left) but not neurons treated with emetine (right). Image credit: MacGibeny et al., 2018.

Enquist and colleagues previously found that alpha herpesviruses engage the neuronal transport machinery by stimulating protein synthesis at infected nerve terminals. Viral transport to the cell body can therefore be blocked by drugs that inhibit protein synthesis, as well as by cellular antiviral proteins called interferons.

In the current study, Enquist and colleagues investigated how the rabies virus engages the neuronal transport machinery. The researchers infected neurons with a virulent strain of the virus tagged with a red fluorescent protein, allowing the researchers to observe viral transport in real time by live-cell fluorescence microscopy.

The study was led by Margaret MacGibeny, who earned her Ph.D. in 2018, and associate research scholar Orkide Koyuncu, at Princeton, with contributions from research associate Christoph Wirblich and Matthias Schnell, professor and chair of microbiology and immunology at Thomas Jefferson University.

In contrast to alpha herpesvirus infections, the team found that interferons had no effect on rabies virus transport, perhaps because, until it reaches the neuronal cell body, the rabies virus hides out inside cellular structures called endosomes.

“We also couldn’t detect increased protein synthesis in axons upon rabies virus infection,” MacGibeny said. “But, to our surprise, we saw that a protein synthesis inhibitor called emetine efficiently blocked rabies virus transport to the cell body.”

Emetine had no effect on the transport of endosomes devoid of the rabies virus. But endosomes carrying the virus were either completely immobilized, or were only able to move short distances at slower-than-normal speeds.

Other protein synthesis inhibitors did not block rabies virus transport, however, suggesting that emetine works by inhibiting a different process in infected neurons.

“Emetine has been used to treat amoebic dysentery,” Koyuncu said. “In the laboratory it is widely used to inhibit protein synthesis but there are recent reports indicating that emetine has anti-viral effects that are independent of protein synthesis inhibition. Our study shows that this drug can inhibit rabies virus invasion of the nervous system through a novel mechanism that hasn’t been reported before.”

“The manuscript by MacGibeny et al. both advances and complicates our understanding of how neurotropic viruses make their way from the axon terminus to the cell body,” said Professor Glenn Rall, an expert in neurotropic virus infections at Fox Chase Cancer Center, who was not involved in the study. “Revealing variations in the axonal transport of neurotropic viruses, coupled with intriguing insights into new roles for well-known drugs, has both mechanistic and clinical implications for these life-threatening infections.

“Our next step is to figure out how emetine disrupts rabies virus transport in axons,” Enquist says. “Does it inhibit cell signaling pathways after rabies virus entry, or does it directly block the recruitment of motor proteins to virus-carrying endosomes?”

This study was funded by the US National Institutes of Health (grants P40 OD010996, RO1 NS33506, and F30 NS090640).

The study, Retrograde axonal transport of rabies virus is unaffected by interferon treatment but blocked by emetine locally in axons, by Margaret A. MacGibeny, Orkide O. Koyuncu, Christoph Wirblich, Matthias J. Schnell, and Lynn W. Enquist was published in PLoS Pathogens. 2018. DOI: 10.1371/journal.ppat.1007188

Text courtesy of the Department of Molecular Biology

A valley so low: Electrons congregate in ways that may be useful to “valleytronics”

Yellow swirling ellipses against a blue background

A Princeton University-led study has revealed an emergent electronic behavior on the surface of bismuth crystals that could lead to insights on the growing area of technology known as “valleytronics.”

The term refers to energy valleys that form in crystals and that can trap single electrons. These valleys potentially could be used to store information, greatly enhancing what is capable with modern electronic devices.

In the new study, researchers observed that electrons in bismuth prefer to crowd into one valley rather than distributing equally into the six available valleys. This behavior creates a type of electricity called ferroelectricity, which involves the separation of positive and negative charges onto opposite sides of a material. The study was published in the journal Nature Physics.

The finding confirms a recent prediction that ferroelectricity arises naturally on the surface of bismuth when electrons collect in a single valley. These valleys are not literal pits in the crystal but rather are like pockets of low energy where electrons prefer to rest.

The researchers detected the electrons congregating in the valley using a technique called scanning tunneling microscopy, which involves moving an extremely fine needle back and forth across the surface of the crystal. They did this at temperatures hovering close to absolute zero and under a very strong magnetic field, up to 300,000 times greater than Earth’s magnetic field.

The behavior of these electrons is one that could be exploited in future technologies. Crystals consist of highly ordered, repeating units of atoms, and with this order comes precise electronic behaviors. Silicon’s electronic behaviors have driven modern advances in technology, but to extend our capabilities, researchers are exploring new materials. Valleytronics attempts to manipulate electrons to occupy certain energy pockets over others.

The existence of six valleys in bismuth raises the possibility of distributing information in six different states, where the presence or absence of an electron can be used to represent information.  The finding that electrons prefer to cluster in a single valley is an example of “emergent behavior” in that the electrons act together to allow new behaviors to emerge that wouldn’t otherwise occur, according to Mallika Randeria, the first author on the study and a graduate student at Princeton working in the laboratory of Ali Yazdani, the Class of 1909 Professor of Physics.

“The idea that you can have behavior that emerges because of interactions between electrons is something that is very fundamental in physics,” Randeria said. Other examples of interaction-driven emergent behavior include superconductivity and magnetism.

In addition to Randeria, the study included equal contributions from Benjamin Feldman, a former postdoctoral fellow at Princeton who is now an assistant professor of physics at Stanford University, and Fengcheng Wu, a postdoctoral researcher at Argonne National Laboratory. Additional contributors at Princeton were Hao Ding, a postdoctoral research associate in physics, and András Gyenis, a postdoctoral research associate in electrical engineering; Ji Huiwen, who earned a doctoral degree at Princeton and is now a postdoctoral researcher at the University of California-Berkeley; Robert Cava, Princeton’s Russell Wellman Moore Professor of Chemistry; and Yazdani. Additional contributions came from Allan MacDonald, professor of physics at the University of Texas-Austin.

The study was funded by the Gordon and Betty Moore Foundation as part of the EPiQS initiative (GBMF4530), the U.S. Department of Energy (DOE-BES grant DE-FG02-07ER46419), the U.S. Army Research Office MURI program (W911NF-12-1-046), the National Science Foundation’s MRSEC program through the Princeton Center for Complex Materials (NSF-DMR-142054 and NSF-DMR-1608848), and the Eric and Wendy Schmidt Transformative Technology Fund at Princeton. Work at University of Texas-Austin was supported by DOE grant (DE-FG03-02ER45958) and by the Welch Foundation (TBF1473).

The study “Ferroelectric quantum Hall phase revealed by visualizing Landau level wave function interference,” by Mallika T. Randeria, Benjamin E. Feldman, Fengcheng Wu, Hao Ding, András Gyenis, Huiwen Ji, R. J. Cava, Allan H. MacDonald, and Ali Yazdani, was published online May 14, 2018, and in print in August, 2018, in the journal Nature Physics.

By Catherine Zandonella