This work grew out of a longstanding interest in the synaptic effects of Aβ, and how they might translate into network effects (see, e.g., Mucke et al., 2000 and Palop and Mucke, 2010). Previous work in the field showed that fewer plaques formed in the mouse hippocampus when scientists severed its connection with the entorhinal cortex, a region where Aβ pathology takes hold early in AD (see ARF related news story on Lazarov et al., 2002 and Sheng et al., 2002). Nonetheless, these earlier experiments provided no direct evidence for the transport of Aβ through synapses, since the effects could have been caused by loss of electrical connections. Other researchers in the field say these new results now supply compelling data that Aβ can be released from synapses into neighboring brain regions, and that this process relates to the spread of disease through the brain. The study was not designed to test the idea that Aβ can propagate in a prion-like way, which is currently a hot topic in the field, but the data leave the door open for such a mechanism, commentators say. The findings also suggest the entorhinal cortex might be a target for early therapeutic interventions that might one day help prevent Alzheimer’s from running wild through the brain.

“This is a great follow-up to [the earlier] studies,” said John Cirrito of the University of Washington in St. Louis, Missouri. The paper shows that “it really is material being transported down the perforant path and being released in the hippocampus that is causing those plaques to be formed.”

“What I like about [the paper] is that they are showing this trans-synaptic progression in an intact system in live mice,” said Lary Walker of Emory University in Atlanta, Georgia. “It is a very elegant approach.”

The entorhinal cortex (EC), part of the brain’s memory formation system, interests scientists because numerous pathological and imaging studies have shown that it is one of the first areas to succumb to AD (see Braak and Braak, 1991; Gomez-Isla et al., 1996; and Wu and Small, 2006).

The EC develops tau tangles and loses neurons early in AD. The disease has been found to selectively wither interconnected brain networks (see ARF related news story on Seeley et al., 2009), suggesting that pathology spreads from a source, such as the EC, via synaptic connections. The EC connects directly to the dentate gyrus of the hippocampus, and scientists have shown that APP made in the EC gets transported down the axons of the perforant pathway and accumulates at pre-synaptic terminals in the dentate gyrus (see Buxbaum et al., 1998). Scientists also know that Aβ can be released from synapses. Researchers led by Cirrito and David Holtzman at Washington University demonstrated that synaptic activity leads to endocytosis and cleavage of APP and the release of Aβ (see ARF related news story on Cirrito et al., 2008). More recently, a team led by Roberto Malinow at the University of California in San Diego showed that Aβ can be released both pre-synaptically and post-synaptically (see ARF related news story on Wei et al., 2010).

To tie these threads together conclusively, first author Julie Harris in Mucke’s lab developed a mouse model (EC-APP) that limits overexpression of human APP containing familial AD Swedish and Indiana mutations to the entorhinal cortex. She created the mice by crossing tet-APP mice, in which mutant APP is under the control of a tetracycline-inducible promoter, with neuropsin-tTA mice (see Yasuda and Mayford, 2006) that express tetracycline primarily in the EC. Harris and colleagues verified by immunostaining that these mice initially have high levels of mutant APP in the EC, but very little elsewhere. At six months of age, EC-APP mice have Aβ deposits in the EC, but the dentate gyrus remains clear. By 13 months, however, numerous Aβ accumulations dot the hippocampus, primarily in the areas where the perforant pathway terminates. This suggests that mutant protein travels from the EC and pops out of synapses into the hippocampus.

Harris and colleagues also examined numerous behavioral, electrical, and biochemical features of the EC-APP mice. The authors found that they share many of the AD-like symptoms of hAPP-J20 mice that produce mutant APP throughout their brains. This adds further evidence that seeding pathology into the EC can initiate the spread of disease into other brain regions. For example, EC-APP mice are less anxious than normal mice in the elevated plus maze, and as they age they develop learning problems in the Morris water maze and become hyperactive in the Y-maze, just like their hAPP-J20 cousins. EC-APP mice experience cortical electrical storms, similar to other mutant APP lines as well (see ARF related news story on Palop et al., 2007).

At the molecular level, EC-APP and hAPP-J20 mice develop similarly. Compared to normal mice, they produce less calbindin and Fos in the dentate gyrus. Loss of these proteins correlates with memory problems (see ARF related news story on Palop et al., 2003). As they age, EC-APP mice, like hAPP-J20 mice, lose the synaptic protein synaptophysin, and have weaker long-term potentiation in the dentate gyrus, further indications that disease has spread from the EC to the hippocampus. To demonstrate that Aβ itself was causing these behavioral impairments, Harris and colleagues treated four-month-old EC-APP mice with a γ-secretase inhibitor for two days to lower Aβ levels. Treated mice behaved like wild-type mice in the elevated plus maze.

The authors’ next interest is to find out how Aβ produces harmful effects, Harris said. Is it through a direct action of Aβ on hippocampal neurons, or does Aβ influence EC neurons to change their output properties and thus indirectly affect the hippocampus? To answer this, Harris said, they will try to remove Aβ from the hippocampus of the EC-APP mice, perhaps by using viral vectors to introduce a protein that degrades Aβ. If various behavioral and biochemical disease symptoms go away, this would suggest that harmful alterations are directly caused by Aβ acting in the dentate gyrus.

“One of the strengths of the paper is that they looked at a lot of different endpoints,” Walker said, praising the inclusion of behavioral and electrical measures. Walker also points out that the hippocampal alterations in EC-APP mice correlate with where Aβ deposits form, implying that Aβ is causing these symptoms. Cirrito agrees that the new data, in combination with previous work in the field, provide excellent evidence for the release of Aβ from EC neurons into the dentate gyrus. The fact that synaptically released Aβ can then glom together into plaques is an intriguing new finding, Cirrito said. Cirrito also suggested, based on previous studies, that APP rather than Aβ is transported down the axons, and is then processed into Aβ at synapses in response to activity, although he said the current paper does not directly show this.

A hot topic in neurodegenerative research right now is the idea that pathogenic proteins can spread in a prion-like fashion, with a small infusion of mutant proteins able to seed aggregates throughout the brain. Several studies have shown that tau can behave this way (see Frost et al., 2009 and ARF related news story on Clavaguera et al., 2009). Aβ has the same ability (see ARF related news story on Eisele et al., 2009). Since the study by Harris and colleagues did not set out to test the idea of a prion-like propagation mechanism, Cirrito said, it is hard to draw conclusions about prion-like behavior in this system. The authors show that Aβ forms extracellular plaques in the dentate gyrus, he said, but it is not clear whether Aβ is also taken up by the post-synapse to cause aggregation within another cell. Walker agrees that it is too early to say whether prion-like transmission is a factor here. “Is a misfolded form of Aβ being transferred to another cell and then causing the Aβ there to misfold? That’s not definitely shown here, but it certainly suggests that this may be the mechanism that’s involved.”

The findings imply that the EC might make a good therapeutic target during the earliest stages of cognitive impairment. “I think our study could help to guide the decision about where in the brain therapy needs to be introduced,” Harris said. The introduction of brain-derived neurotrophic factor into the EC has been shown to improve AD symptoms in animal models (see ARF related news story on Nagahara et al., 2009). Walker sees another approach. “If there are pathogenic seeds that are being transferred between cells,” he said, then “if you could introduce a small molecule into the brain that would mop up these seeds, you might be able to minimize the spread [of the disease] from one region to another.”—Madolyn Bowman Rogers.

References:
Harris JA, Devidze N, Verret L, Ho K, Halabisky B, Thwin MT, Kim D, Hamto P, Lo I, Yu GQ, Palop JJ, Masliah E, Mucke L. Transsynaptic progression of amyloid-beta-induced neuronal dysfunction within the entorhinal-hippocampal network. Neuron. 2010 Nov 4;68(3):428-41. Abstract