1 March 2013. Shutting off transgenic Aβ production in middle-aged mice whose brains are jam-packed with amyloid pathology was able to rescue some aspects of cognition, according to a study in the February 27 Journal of Neuroscience. In this first publication of behavioral phenotypes in an Alzheimer's mouse model expressing tetracycline-regulated amyloid precursor protein (APP), researchers report that suppressing APP restored spatial and working memory, even while brain amyloid deposits and soluble Aβ42 remained. "At this very advanced stage of amyloidosis, we could still rescue some part of cognition," said Alena Savonenko of Johns Hopkins University School of Medicine, Baltimore, Maryland, who led the study with David Borchelt of the University of Florida, Gainesville. While some scientists say the work sustains hope that therapies regulating APP processing might be able to help people with advanced amyloidosis, others question the human relevance of data obtained in a mouse model with such rampant plaque pathology.
To address what happens when Aβ production is blocked after plaques form, Borchelt, Savonenko, and colleagues created a mouse in which they could suppress forebrain expression of a mutant APP transgene by adding doxycycline (DOX), a tetracycline derivative, to the animals' diet. Back in 2005, the scientists reported that turning off APP expression for three to six months kept Aβ pathology from progressing but did not dissolve existing plaques in these mice (see ARF related conference story and Jankowsky et al., 2005).
In the current paper, first author Tatiana Melnikova of Johns Hopkins and colleagues report the long-awaited cognitive data in this model. Why did it take so long? The inducible APP mice from the 2005 paper were unusually hyperactive; their constant scampering interfered with behavioral tasks that use novelty-induced motor activity as a readout for memory. Reducing the hyperactivity required changes in breeding strategy and DOX protocol. The scientists bred the APP and tetracycline-transactivator (tTA) transgenic lines 10 generations onto C57BL/6J and FVB/N backgrounds, respectively, then mated these to create double-transgenic F1 hybrids. (In tTA mice, behavior varies with strain background [see Han et al., 2012], and this particular combination seemed to work best.) The second "trick" was providing doxycycline during the first four to six weeks of life, instead of letting the animals develop DOX-free from birth as they had done in the 2005 study. Putting the animals on a DOX diet during their first few weeks of life "delayed expression of the transgene, allowing the mice to develop normally without too much APP flying around," Savonenko said.
After four to six weeks on DOX, the scientists switched the mice to DOX-free chow, unleashing APP expression until 12 to 13 months of age, by which time the mice had advanced amyloidosis. DOX was then reintroduced to shut off the transgene. "The whole cortex is flooded with plaques. In the hippocampus, you can barely see the structure," Savonenko said.
Yet after just seven days on DOX, the mice performed as well as non-transgenic and vehicle-treated controls in tests of short- and long-term spatial memory and working memory. The DOX-treated animals still struggled with tasks requiring episodic memory, and had more trouble adjusting to changes in experimental paradigms than did control mice.
As expected based on the 2005 findings, suppressing APP did not seem to influence Aβ load. "Everything we can see that's related to Aβ—soluble peptide, Aβ-immunoreactive species that run in gels the size of oligomers, amyloid plaques—all those things don't change when you turn the transgene off," Borchelt said. On the other hand, full-length APP, soluble APP ectodomains, and C-terminal APP fragments nearly disappeared after shutting off the transgene.
"This study clearly indicates that Aβ burden could be dissociated from cognitive decline associated with AD," Angèle Parent of the University of Chicago, Illinois, wrote in an e-mail to Alzforum. "If we could extrapolate these results to AD patients, it suggests that reducing expression of 'bad' APP (and its β C-terminal fragment accumulation) may be sufficient to improve cognitive function in a relatively short period of time." Recent work from her lab and others suggests that non-Aβ APP fragments play key roles in neuron health (see ARF related news story).
The data are curious in that memory improved in DOX-treated mice despite unchanged levels of what could be Aβ oligomers, which are widely seen as the most neurotoxic molecules in AD. However, the authors and other scientists were not certain they were measuring Aβ oligomers in this study. Though their immunoblots showed PBS-insoluble structures that ran around the same size as synthetic Aβ oligomers, "we could not easily detect soluble oligomers. If they are there, the levels are very low," Borchelt said. Nevertheless, those Aβ species persisted, and yet the mice got better.
Karen Ashe and Peng Liu of the University of Minnesota, Minneapolis, propose that the reduced levels of transgenic APP may contribute to the memory improvement in the mice, which have abundant neuritic plaques. The scientists have submitted a paper suggesting that neuritic plaques are neurotoxic "because the Aβ fibrils in the plaque core are a source of oligomers that diffuse away from the core," and that plaques can "act like space-occupying lesions in the brain and disrupt function," they wrote to Alzforum (see also Ashe and Aguzzi, 2013). In another submitted manuscript, Ashe and Liu show that oligomers within neuritic plaques require APP to exert their toxic effects. "DOX-mediated suppression of transgenic APP may block the APP-Aβ oligomer interaction, in turn ameliorating memory deficits," they suggest.
Another question is how well the inducible APP mice model AD, because their neuritic plaque load well exceeds that attained in the human disease, Ashe and Liu noted (see full comment below). Savonenko and Borchelt said the team is currently studying the effects of transgenic APP suppression in younger mice with less advanced amyloidosis. They are also exploring whether intracellular location of the presumed Aβ oligomers influences cognition in this model.—Esther Landhuis.
Melnikova T, Fromholt S, Kim H, Lee D, Xu G, Price A, Moore BD, Golde TE, Felsenstein KM, Savonenko A, Borchelt DR. Reversible pathologic and cognitive phenotypes in an Inducible model of Alzheimer-amyloidosis. J Neurosci. 27 Feb 2013;33(9):3765-3779. Abstract