Does Aβ, the quintessential Alzheimer’s disease protein, multiply from a few seeds and spread through brain networks like kudzu taking over a garden patch? This idea has been taking root for some time, with researchers hypothesizing that the pathogenic protein gets carted down axons and strewn into unspoiled brain regions. Supporting this idea, researchers led by Lennart Mucke at the University of California in San Francisco reported in yesterday’s Neuron that Aβ pathology can invade new areas of the brain through synaptic connections. To demonstrate this, Mucke and colleagues developed a mouse model that overexpresses mutant APP, the precursor protein of Aβ, primarily in the entorhinal cortex. As the mice age, they develop Aβ plaques and other AD-like changes in the regions of the hippocampus that receive inputs from the entorhinal cortex, supporting the idea that Aβ disperses through connecting pathways.

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

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  1. I’ve read the article and find it to be interesting and to provide important new information. It demonstrates that overproduction of β amyloid in the entorhinal cortex can lead to synaptic aberrations in the hippocampus. This supports the view that pre-synaptic Aβ can lead to synaptic abnormalities. It would have been interesting to measure synaptic properties in EC neurons, particularly in regions where they receive afferent input from non-overexpressing regions, to see if they are equally or more affected, than hippocampal neurons. This could address if pre-synaptic or post-synaptic Aβ has more deleterious effects. From the deposition data they show, the EC appears to be more affected, suggesting that cell body and dendritic Aβ may be more abundant.

    The mechanism by which this ”trans-synaptic” effect is transmitted will be interesting to identify.

    View all comments by Roberto Malinow
  2. The paper by Harris et al. is right on target with regard to the nucleation of plaques by seeds of oligomeric Aβ that ultimately could come from cells within brain regions remote from the site of plaque formation. The favored view appears to be that the Aβ that accumulates at the terminal fields of the perforant pathway is produced locally—in pre-synaptic endosomes (1) from axonally transported full-length APP, or C-terminal fragments (CTFs) of APP (2-4). Yet a large fraction of Aβ is generated in the neuronal soma, in the endoplasmic reticulum, the trans-Golgi network, and recycling endosomes. We (5), and others (6) consistently detected Aβ-positive, vesicle-like particles along neuronal processes, suggesting that at least some of the Aβ within terminals could come from transport of the cleaved polypeptide, generated in the soma. In any case, the precursor of Aβ, or the Aβ itself, which nucleates the hippocampal deposits in the mouse models described by Harris et al., is indeed brought from the distance.

    We recently proposed a mechanism for nucleation of plaques within the cortex and hippocampus; the seeding, oligomeric Aβ is produced remotely, in the soma of brainstem neurons that project into the cortex and hippocampus (7,8). Working with cells in culture, we showed that locus coeruleus-derived neurons are particularly prone to producing intracellular Aβ oligomers that accumulate at the terminals of their processes (9). We also showed that these neuritic Aβ aggregates can become extracellular (7). While our view is that a fraction of the plaques in the cortex and hippocampus of Alzheimer’s disease (AD) brain is nucleated by Aβ oligomers released from the terminals of brainstem neurons, it is certain that Aβ seeds could originate from other brain regions as well. The study by Harris et al. points to one of them—the entorhinal cortex.

    Much remains to be done to prove that such mechanisms indeed occur in the human brain in AD. Yet it is already the time to think that—in AD—treating a brain region remote from the sites of plaque formation may not be as unrealistic as it may seem. Yes, as we spelled it out in one of our recent papers, and is clearly demonstrated by Harris et al., plaques can be seeded from the distance—a very long distance.

    See also: Muresan, Z. and V. Muresan, Brainstem Neurons Are Initiators of Neuritic Plaques. SWAN Alzheimer Knowledge Base. Alzheimer Research Forum

     

    References:

    . Endocytosis is required for synaptic activity-dependent release of amyloid-beta in vivo. Neuron. 2008 Apr 10;58(1):42-51. PubMed.

    . Alzheimer amyloid protein precursor in the rat hippocampus: transport and processing through the perforant path. J Neurosci. 1998 Dec 1;18(23):9629-37. PubMed.

    . Evidence that synaptically released beta-amyloid accumulates as extracellular deposits in the hippocampus of transgenic mice. J Neurosci. 2002 Nov 15;22(22):9785-93. PubMed.

    . Disruption of corticocortical connections ameliorates amyloid burden in terminal fields in a transgenic model of Abeta amyloidosis. J Neurosci. 2002 Nov 15;22(22):9794-9. PubMed.

    . The cleavage products of amyloid-beta precursor protein are sorted to distinct carrier vesicles that are independently transported within neurites. J Neurosci. 2009 Mar 18;29(11):3565-78. PubMed.

    . Intraneuronal beta-amyloid accumulation and synapse pathology in Alzheimer's disease. Acta Neuropathol. 2010 May;119(5):523-41. PubMed.

    . Seeding neuritic plaques from the distance: a possible role for brainstem neurons in the development of Alzheimer's disease pathology. Neurodegener Dis. 2008;5(3-4):250-3. PubMed.

    . Neuritic deposits of amyloid-beta peptide in a subpopulation of central nervous system-derived neuronal cells. Mol Cell Biol. 2006 Jul;26(13):4982-97. PubMed.

  3. The paper by Julie Harris and colleagues is an important contribution toward understanding the role of synaptic networks in progression of neuronal dysfunction and Aβ deposition. They produced and studied transgenic mouse models with region-specific overexpression of mutant APP in the entorhinal cortex (EC) layer II/III neurons, and have shown that Aβ deposition occurs in the terminal projection zones of these neurons, and that functional impairments can cross synapses. In this model, such abnormalities occur initially in the EC neurons and extend to the hippocampal cells. As the authors mentioned, the EC is one of the earliest affected regions in AD. It has to be noted, however, that in humans, the early pathology takes the form of neurofibrillary tangles, which are composed of abnormally phosphorylated tau protein (Braak and Braak, 1991). It is well known that tau abnormalities precede Aβ deposition in this area in AD.

    There is increasing evidence that intracellular accumulation of abnormal proteins such as tau and α-synuclein may be transferred from cell to cell, propagating by a prion-like mechanism (Goedert et al., 2010; Nonaka et al., 2010). It will be interesting to see whether abnormal intracellular proteins propagate through the synapses in similar transgenic mouse models that selectively overexpress mutant tau or α-synuclein in areas where they first accumulate in diseases.

    View all comments by Masato Hasegawa

References

News Citations

  1. Can Travel, Will Deposit: Aβ via the Perforant Pathway?
  2. Network Connections: Missing Links in Neurodegeneration?
  3. Link Between Synaptic Activity, Aβ Processing Revealed
  4. Aβ—Made Globally, Acts Locally
  5. Do "Silent" Seizures Cause Network Dysfunction in AD?
  6. Calbindin Study: Is Calcium the Molecular Handle on Dysfunction in AD?
  7. Traveling Tau—A New Paradigm for Tau- and Other Proteinopathies?
  8. Aβ the Bad Apple? Seeding and Propagating Amyloidosis
  9. BDNF the Next AD Gene Therapy?

Paper Citations

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  2. . Amyloid-beta-induced neuronal dysfunction in Alzheimer's disease: from synapses toward neural networks. Nat Neurosci. 2010 Jul;13(7):812-8. PubMed.
  3. . Evidence that synaptically released beta-amyloid accumulates as extracellular deposits in the hippocampus of transgenic mice. J Neurosci. 2002 Nov 15;22(22):9785-93. PubMed.
  4. . Disruption of corticocortical connections ameliorates amyloid burden in terminal fields in a transgenic model of Abeta amyloidosis. J Neurosci. 2002 Nov 15;22(22):9794-9. PubMed.
  5. . Profound loss of layer II entorhinal cortex neurons occurs in very mild Alzheimer's disease. J Neurosci. 1996 Jul 15;16(14):4491-500. PubMed.
  6. . Imaging the earliest stages of Alzheimer's disease. Curr Alzheimer Res. 2006 Dec;3(5):529-39. PubMed.
  7. . Neurodegenerative diseases target large-scale human brain networks. Neuron. 2009 Apr 16;62(1):42-52. PubMed.
  8. . Alzheimer amyloid protein precursor in the rat hippocampus: transport and processing through the perforant path. J Neurosci. 1998 Dec 1;18(23):9629-37. PubMed.
  9. . Endocytosis is required for synaptic activity-dependent release of amyloid-beta in vivo. Neuron. 2008 Apr 10;58(1):42-51. PubMed.
  10. . Amyloid beta from axons and dendrites reduces local spine number and plasticity. Nat Neurosci. 2010 Feb;13(2):190-6. PubMed.
  11. . CaMKII activation in the entorhinal cortex disrupts previously encoded spatial memory. Neuron. 2006 Apr 20;50(2):309-18. PubMed.
  12. . Aberrant excitatory neuronal activity and compensatory remodeling of inhibitory hippocampal circuits in mouse models of Alzheimer's disease. Neuron. 2007 Sep 6;55(5):697-711. PubMed.
  13. . Neuronal depletion of calcium-dependent proteins in the dentate gyrus is tightly linked to Alzheimer's disease-related cognitive deficits. Proc Natl Acad Sci U S A. 2003 Aug 5;100(16):9572-7. Epub 2003 Jul 24 PubMed.
  14. . Conformational diversity of wild-type Tau fibrils specified by templated conformation change. J Biol Chem. 2009 Feb 6;284(6):3546-51. PubMed.
  15. . Transmission and spreading of tauopathy in transgenic mouse brain. Nat Cell Biol. 2009 Jul;11(7):909-13. PubMed.
  16. . Induction of cerebral beta-amyloidosis: intracerebral versus systemic Abeta inoculation. Proc Natl Acad Sci U S A. 2009 Aug 4;106(31):12926-31. PubMed.
  17. . Neuroprotective effects of brain-derived neurotrophic factor in rodent and primate models of Alzheimer's disease. Nat Med. 2009 Mar;15(3):331-7. PubMed.

Other Citations

  1. Braak and Braak, 1991

Further Reading

Primary Papers

  1. . Transsynaptic progression of amyloid-β-induced neuronal dysfunction within the entorhinal-hippocampal network. Neuron. 2010 Nov 4;68(3):428-41. PubMed.