2 December 2005. This is the final installment of a four-part news series about the role of intraneuronal Aβ from the 35th Annual Conference of the Society for Neuroscience, held November 12 to 16 in Washington, D.C. See also Introduction and Part 1, Part 2, and Part 3.
Aβ Jams Communication Inside Cell
Besides intracellular trafficking and synaptic activity, there are other areas of cell biology with which intraneuronal Aβ seems to interfere. Henry Querfurth’s group, of St. Elizabeth’s Medical Center in Boston, presented a series of data on how intraneuronal Aβ disrupts neuroprotective signaling and hampers the cell’s stress response. Querfurth reported at the conference, and also in the November 23 Journal of Neuroscience, that intraneuronal Aβ impinges on the phosphatidyl inositol-3 (PI3K)-Akt pathway (Magrane et al., 2005). The serine/threonine protein kinase B known as Akt is established as playing an important role in neuronal survival in normal brain and known to malfunction in conditions ranging from ALS to Huntington disease and schizophrenia. Some labs have even suggested that FAD mutations depress the PI3K-Akt pathway (see, e.g., Ryder et al., 2004). Other bits and pieces of circumstantial evidence for a role of Akt in AD exist in the literature, but they are disparate and do not focus expressly in intraneuronal Aβ.
Using an inducible viral vector system, Jordi Magrane, formerly in Querfurth’s lab, had reported earlier that intraneuronal Aβ42 expression is toxic to cultured neurons (Magrane et al., 2004). This time, Magrane examined the peptide’s possible effect on Akt. Akt itself needs to be phosphorylated to become active, and then it phosphorylates a number of target proteins that go on to perform various neuroprotective functions. Magrane found that intracellular, but not extracellular, Aβ reduced this Akt activation in rat primary cortical neurons. Active Akt puts a damper on the tau kinase GSK3β; in the Aβ-expressing neurons, impaired Akt meant that GSK3β was more active, hinting at a link between Aβ and tau through this channel. The frontal cortex of Tg2576 mice, too, showed reduced phosphorylated Akt compared to wild-type mice at the time that intraneuronal Aβ begins to show up in this model.
On the flip side, overexpressing Akt inside primary neuronal cultures protected the cells from the toxic effects of accumulating intraneuronal Aβ, Querfurth reported. GSK3β is but one downstream target of Akt. In experiments studying another, the scientists found that active Akt induces the chaperone HSP70 and in this way mobilizes the cell’s response to stress. Subsequent siRNA experiments suggested that HSP70, in turn, acts downstream of Akt to protect cells from intraneuronal Aβ.
All together, the scientists suggest that extracellular Aβ, especially synthetic Aβ applied to cultures, may act quite different from intraneuronal Aβ. The inducible adenoviral system this group developed allows them to control the expression of Aβ and then tease out the role of various components and their place in the signal transduction chain, Querfurth said. The researchers believe their system is relevant to what happens in vivo in part because they target Aβ to be expressed in the secretory pathway, mimicking the site where it is made in vivo. It’s still not entirely clear how Aβ throws a wrench into this chain of survival events. Previous work using another cell type had suggested it might interfere with the interaction between Akt and its activator, phosphoinositide-dependent kinase 1 (Suhara et al., 2003), and current research pursues this lead, Querfurth said.
In summary, the scientists propose that in early AD, intraneuronal Aβ might wreak its damage partly by preventing sufficient Akt activation, which would enfeeble the cell’s stress response and steer the cell away from survival signals. Thinking in terms of therapies, this work strengthens the case for neuroprotective strategies trying to induce Akt, as some experimental neuroprotective compounds, such as VEGF, BDNF, and IGF-1, are known to do. For yet a different angle on intraneuronal Aβ presented by this group, see related SfN story on parkin.
Implications for Immunotherapy
Beyond giving a thumbs up to neuroprotective endeavors, the growing body of data on intraneuronal Aβ has implications for ongoing therapeutic approaches in AD. According to Gouras, it raises new questions about how immunotherapies might work. Broadly speaking, scientists studying such therapies consider two hypotheses: Either the antibodies sequester Aβ in the plasma and draw down brain levels indirectly (the peripheral sink hypothesis), or the antibodies cross the blood-brain barrier and bind extracellular Aβ to mark it for ingestion by microglia. Both ideas have experimental support, yet they do not explain why antibodies injected into the brain reduced the levels of intraneuronal Aβ (Billings et al., 2005). Could some therapeutic antibodies even make their way inside neurons? To address this question, Gouras’s group treated APP-transfected neuroblastoma cells and cultured Tg2576 neurons with such antibodies. (Tg2576 cells are a model for this because in these mice’s brains, as in human AD brain, intraneuronal Aβ levels increase with age in multivesicular bodies; see Takahashi et al., 2002.) After a day of incubation with N-terminal anti-Aβ antibodies, the intraneuronal pool of Aβ, and staining in neuronal processes, decreased in both cell types, possibly due to a drop in production, Gouras reported. In these cell cultures, the antibodies not only were internalized into the neurons, but active endocytosis also was necessary for intraneuronal Aβ levels to decrease. Gouras suggests that when the antibody binds to the extracellular, N-terminal domain of Aβ on APP, it might be taken up into the neuron along with APP. Once inside the neuron, the antibody might prevent the effect of Aβ on synaptic and signaling proteins, and in this way ameliorate cognitive deficits independent of extracellular amyloid pathology.
Gouras noted that this new proposed mechanism does not exclude others. Depending on the nature of the immunotherapy (active, passive, choice of antibody, etc.), different mechanisms may be at play. Gouras’s “immunotherapy” of the cultured cells, and also LaFerla’s injections of the triple transgenic mice, used antibody concentrations higher than what may be easily achievable in the brain following peripheral injection. These concentrations were useful to delineate the cellular pathway; testing the in-vivo relevance of this work will be a next step.
In summary, a small but growing chorus of scientists maintains that intraneuronal Aβ, not the extracellular pool that grows at a later stage, plays an early pathogenic role in various AD models, and they speculate that the same process occurs in AD. Extracellular Aβ and amyloid deposition, they purport, later compound the early problems.—Gabrielle Strobel.