British researchers report in today’s Science that they have managed to prevent and even reverse clinical manifestations of prion disease. Surprisingly, they did it not by eliminating deposits of abnormal prions, but by depleting normal prion protein. Beyond the therapeutic implications, this result suggests that neurotoxicity in the disease is a function of the protein’s conversion process rather than its final product.
Prion diseases ranging from Creutzfeldt-Jacob to mad-cow are caused by prion proteins (PrPc) being misfolded, or so the story goes. But do these abnormal proteins (PrPSc) cause neurodegeneration? There are some bits of evidence to the contrary, even aside from the fact that some human prion diseases feature little or no detectable PrPSc, the authors write. For example, the accumulation of PrPSc does not necessarily correlate with disease symptoms in animal models, and the addition of PrPSc to brain tissue that does not also have endogenous or transgenic PrPc will not induce neurodegeneration in this tissue.
Still, the conversion of PrPc to some other form appears to be critical for the disease, so John Collinge, Giovanna Mallucci, and colleagues at the Institute of Neurology in London decided to target PrPc directly. The researchers generated mice transgenic for a prion protein gene that increases the amounts of PrPc produced, but also harboring a transgene for the enzyme Cre recombinase, which shuts down the expression of the PrP transgenes. The beauty of this experimental system is that a neurofilament heavy chain promoter does not allow Cre to be expressed until the mice are 10 to 12 weeks of age, and limits expression to neurons. Thus, the mice overproduce PrPc in both neurons and glia until about 12 weeks of age, after which Cre shuts this expression down in neurons.
The experiment began with the inoculation of mice with scrapie prions at three to four weeks of age. At 12 weeks of age, both PrP and Cre/PrP transgenic animals show pathological evidence of infection, though they still have minimal spongiosis and no behavioral signs of disease. At that point, PrP transgenic mice (lacking the Cre enzyme to shut down PrP in neurons) proceed to develop spongiosis and die around 20 weeks of age.
By contrast, at last count, mice with the Cre gene were still going strong—and asymptomatic—at an average 60 weeks of age. This survival correlates with a shutdown of PrPc in neurons, but not in glia, and a rescue of neurons from degeneration. Surprisingly, however, PrPSc continues to accumulate in these asymptomatic mice. The reason appears to be that glial conversion of PrP is not shut down. What’s more, the little bit of spongiosis that had begun in mice at 12 weeks of age was reversed when Cre expression kicked in.
"Our results…argue against direct neurotoxicity of PrPSc," write the authors. They add that "[I]t appears that the conversion of PrPc to disease-related forms must occur within neurons to be pathogenic, consistent with the possibility that a toxic intermediate is generated within neurons during the conversion process."—Hakon Heimer