17 August 2009. Chances are, when asked to list possible causes for Parkinson and Alzheimer diseases, many scientists would not think to include flu virus. Yet a study in this week’s PNAS Early Edition lifts the so-called “pathogen hypothesis” off the backburner. Led by Richard Smeyne, researchers at St. Jude Children’s Research Hospital in Memphis, Tennessee, report that a deadly bird flu virus can reach the central nervous system of infected mice, triggering neuroinflammation and PD-like pathology.
“I think it was a well-done study, and I think the data do suggest that this type of influenza virus can cause the damage we would see with Parkinson's,” said Brian Balin, who studies diseases of aging at the Philadelphia College of Osteopathic Medicine, Pennsylvania. “It fits with what a lot of people have been doing, or have intimated over the years—that infection can cause a neurodegenerative process.” (For reviews, see Mattson, 2004; Itzhaki et al., 2004; Balin et al., 2008).
Motivated in part by controversial epidemiological data linking neurological problems with past flu outbreaks such as the devastating 1918 “Spanish” flu (Maurizi, 1985; Ravenholt and Foege, 1982), the project got off the ground after a lunchtime chat between two postdocs. Three years ago, when people feared the H5N1 bird flu virus would become pandemic, Kennie Shepherd of Smeyne’s group and a postdoc working with influenza scientist Robert Webster suggested that their labs pool resources to ask what H5N1 could do in the nervous system. Smeyne’s lab had the neurodegenerative expertise, Webster’s the flu virus. Around the same time, Smeyne had had an “aha” moment while viewing video footage of H5N1-infected geese from Webster’s research expedition in Laos. “Those birds had trouble initiating movement. They'd fall side to side, and the like,” Smeyne recalled. “I said to myself, ‘they look like they have Parkinson disease.’”
For the current study, first author Haeman Jang and colleagues infected mice intranasally with a 50 percent mouse-lethal dose of H5N1. “We wanted the mode of inoculation to be the same, and we wanted the mice to get the equivalent dose of influenza that a human would be exposed to,” Smeyne said. Mimicking human disease patterns, half of the mice died quickly, while the other half got sick and recovered by three weeks post-infection.
In immunohistochemistry studies, the researchers used antibody to the influenza virus nucleoprotein (NP) to trace H5N1’s journey through the nervous system. “Our prediction was that it was going to get into the nervous system through areas where the blood-brain barrier breaks down, but it didn't,” Smeyne said. Instead, H5N1 appeared to reach the nervous system via neurons connecting from the lungs and gut. The virus began at the cranial nerves—first detectable in the vagus nerve, then the nose and olfactory bulb, and onward through the respiratory system.
The progression of H5N1 in mice bears uncanny resemblance to the timing of symptoms in PD patients, many of whom first experience gut motility problems. “They get really badly constipated,” Smeyne said. “Then it progresses to changes in sleep-wake cycles. And then you finally start getting motor symptoms.” Altered sense of smell resulting from damaged olfactory neurons also seems to develop before cardinal symptoms in PD and AD (see ARF related news story). On the pathological side, aggregation of phosphorylated α-synuclein, is believed to precede clinical symptoms in both diseases.
In H5N1-infected mice, the researchers found that the presence of virus seemed to correlate with levels of phosphorylated α-synuclein. “It basically doubled wherever the virus was, and where the virus was not, there was no change at all,” Smeyne said. Infected mice had a 17 percent loss of dopaminergic neurons in the substantia nigra pars compacta (the brain region most affected by neurodegeneration in PD). They also had upregulation of activated microglia, which persisted well beyond day 21, when scientists could no longer detect virus in the brain. “We've looked up to 120 days later, and it looked like it did the day we first saw the active infection,” Smeyne said of the microglial response.
To Smeyne, this implicates a hit-and-run mechanism. “The virus is gone, but you’ve primed the nervous system for other things to affect the brain that ordinarily wouldn’t,” he said. “The infection is sort of like a stealth hit, and we think you need something else to then cause disease.” However, Paul Ewald, an evolutionary biologist at the University of Louisville, Kentucky, offered a more cautious interpretation in an e-mail to ARF. “It has become clear over the past three decades that infectious agents may persist cryptically within their hosts far longer than is suggested by techniques that measure the conspicuous first flush of infection,” he wrote, noting that the scientists’ inability to detect virus at 21 days post-infection does not exclude the possibility that neuronal damage was induced by cryptic persistent infection. (See full comment below.)
Whatever the mechanism, “if uncommon flu strains such as H5N1 can damage dopaminergic neurons, it is worth considering the question of whether common flu strains that spread through the population each year also damage dopaminergic neurons,” wrote clinician-investigator Russell Swerdlow, University of Kansas School of Medicine, Kansas City, in an e-mail to ARF. “If so, flu viruses may contribute more to the development of parkinsonism or perhaps even Parkinson disease than we currently appreciate.”
Other studies suggest that pathogens could be involved in AD, too. CD33, one of four genes found to significantly associate with AD in a recent genomewide association screen (Bertram et al., 2008), is involved in activating the innate immune system. Herpes simplex virus has been found associated with amyloid precursor protein (Satpute-Krishnan et al., 2003 and ARF related news story). But perhaps the most prominent example of a dementia linked to viral infection is human immunodeficiency type 1 (HIV-1)-associated dementia (for a recent review, see Jayadev and Garden, 2009).
Based on his recent findings, Smeyne suspects that West Nile virus, Coxsackie virus, and others that have caused neurological symptoms traverse the nervous system in distinct ways. “Without any experimental proof, I will bet that each virus has a very specific effect on the nervous system,” he said. “It wouldn’t surprise me at all that some viruses affect cholinergic neurons. Then AD becomes important. In our case [H5N1] reached dopaminergic neurons, which are important for PD.”
At this point, it remains unknown whether the new findings are relevant to other flu strains—for example, H1N1, the virus behind the current flu scare. Preliminary evidence had suggested that H1N1 does not reach the nervous system. However, according to a July 24 report from the Centers for Disease Control, there are now four cases in Dallas where H1N1-infected children developed neurological complications. “It could be that the H1N1 virus is mutating and now making itself accessible to the nervous system,” Smeyne said. “Clearly, it would be worthwhile for someone to examine these emerging viruses and ask the simple question, do they get into the nervous system or not?”—Esther Landhuis.
Jang H, Boltz D, Sturm-Ramirez K, Shepherd KR, Jiao Y, Webster R, Smeyne RJ. Highly pathogenic H5N1 influenza virus can enter the central nervous system and induce neuroinflammation and neurodegeneration. PNAS Early Edition. 2009 August. Abstract