Following a recent report linking Fragile X mental retardation protein (FMRP) with translation of amyloid-β precursor protein (see ARF related live discussion), new evidence suggests an additional connection between Alzheimer disease and Fragile X syndrome, the most commonly inherited form of mental retardation. Lowering the activity of p21-activated kinase (PAK), which is decreased in the brains of Alzheimer patients (see ARF related news story), reverses the synaptic and behavioral deficits imposed by the Fragile X gene mutation in mice, according to a paper in the June 25 PNAS online. The finding suggests that inhibiting PAK might be a valuable therapeutic strategy for treating this neurodevelopmental disease, which afflicts—to varying degrees of severity—1 in 4,000 males and 1 in 6,000 females, according to the National Fragile X Foundation.
That cross-breeding the mouse model of Fragile X syndrome (FXS) with transgenic mice with low PAK function would create a seemingly normal mouse may seem too simple. So thought senior author Susumu Tonegawa, at the Massachusetts Institute of Technology, when Mansuo Hayashi, the study’s first author, suggested the idea after she noticed remarkable dendritic spine characteristics in PAK mutants.
“Mansuo looked at the spines of the PAK mutants and found abnormalities that were opposite of what was found in the Fragile X knockouts,” said Tonegawa. In contrast to the spindly, immature spines observed in the brains of the FXS mouse model and in patients with the syndrome, spines in the PAK mutants were stocky. “Mansuo thought it was interesting if you could cross the PAK knockout with the Fragile X mutant, then maybe the effects would cancel out each other,” Tonegawa said.
After cross-breeding the mutants, the researchers quantified and analyzed Golgi-impregnated spines in pyramidal neurons in the temporal cortex. “To my surprise, but maybe not to hers, the mouse with the mutation in both genes had spines that were very normal,” Tonegawa said. “Not too many or too few. Not too thin or too fat. They looked like normal spines in a normal mouse.”
While basal electrophysiological activity did not differ between FXS and PAK/FXS double mutants, there were differences in responses to electrical stimulation. In the double mutant, PAK inhibition reversed deficits in long-term potentiation observed in the FXS-only mutant. In open field and trace conditioning tasks, the researchers also found that the double mutation reversed behavioral abnormalities, including hyperactivity, stereotypy, and attention deficits. The results are the first demonstration of both a cellular and a behavioral reversal of abnormalities in the FXS mouse model.
To test if protein-protein interaction might explain these findings, Hayashi and colleagues performed an immunoprecipitation study, which indicated that PAK and the Fragile X protein existed in a protein complex in vivo. When the researchers purified the proteins and then mixed them in vitro, they bound together.
Because reduction of PAK activity does not occur until the third postnatal week, the study also indicates that PAK inhibition can reverse FXS syndrome after it appears. “Extrapolate this to children 2 or 3 or 4 years old after they are diagnosed, then you can apply this drug therapy and you can reverse it,” Tonegawa said.
Chemical compounds that inhibit PAK exist, although they were not made for that specific purpose. For instance, the pharmaceutical company Cephalon has a PAK inhibitor in development. “It’s not a strong PAK inhibitor and it was developed for a different kinase,” Hayashi said. Evidence from clinical trials suggests that it crosses the blood-brain barrier, she added.
The relationship between PAK signaling and Alzheimer disease remains unclear, although the pathway seems to be impaired in Alzheimer patients. However, the interplay between the enzyme pathway and synapses in both Alzheimer’s and FXS appears more established (see comment from Greg Cole below). “In general, synaptic structure seems to be a common target,” said Hayashi, who is now identifying new drug targets for Alzheimer’s at Merck Research Laboratories in Boston. “Synapse loss seems to precede cognitive loss in AD and FXS.”—Molly McElroy
Molly McElroy is a freelance writer based in Melbourne, Florida.
- Hayashi ML, Rao BS, Seo JS, Choi HS, Dolan BM, Choi SY, Chattarji S, Tonegawa S. Inhibition of p21-activated kinase rescues symptoms of fragile X syndrome in mice. Proc Natl Acad Sci U S A. 2007 Jul 3;104(27):11489-94. PubMed.