Mairet-Coello G, Courchet J, Pieraut S, Courchet V, Maximov A, Polleux F.
The CAMKK2-AMPK kinase pathway mediates the synaptotoxic effects of Aβ oligomers through Tau phosphorylation.
Neuron. 2013 Apr 10;78(1):94-108.
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I would like to point out what I believe constitutes misinterpretations/misconceptions related to oligomeric Aβ found in this article.
In this report, the authors claimed to detect amyloid-β in brain lysates of APP transgenic J20 mice using the antibody 6E10 (Fig. 4). This antibody is specific to the human form of Aβ as well as its precursor molecule APP and some of the proteolytic cleavage products of APP, namely CTF-β. The authors claim that they detected Aβ by Western blot analyses as a band running at ~13 kDa. In fact, monomeric Aβ should be detected at 4 kDa and not at 13 kDa as reported in Figure 4B (unless this represents trimers, but that would require using specific Aβ antibodies or samples which do not contain APP-CTFs to be certain). It is likely that the 6E10 antibody detected the carboxyl terminal fragment of β-secretase-cleaved soluble APP (sAPPβ), which has a molecular weight of 12-13 kDa.
In addition, the histogram (Fig. 4C) illustrating the quantification of these results depicts the levels of human APP and alleged Aβ (which I believe should be labeled as CTF-β) as the percentage of the signal detected in WT mice. Since the non-transgenic mice do not express human proteins, the signal detected with 6E10 in those animals must correspond to noise or non-specific signals, making it impossible to conclude that expression of APP is ~100-fold greater in J20 compared to WT mice. Using 4G8 would be a better choice since it detects both mouse and human APP and its derivatives.
In the absence of liquid phase assays, it is difficult to prove that oligomeric Aβ, and in particular which form oligomeric of Aβ, exists in the animals used in this study. I believe this is a problem that plagues our field, as illustrated by the editorial published in Nature Neuroscience in April 2011 and by the Webinar hosted by Alzforum, which I had the privilege to be part of (ARF Webinar).
Furthermore, the authors show that the AMPK pathway is activated in four-month-old J20 mice compared to wild-type controls. At eight to 10 months, when Aβ deposition is already prominent (Mucke et al., 2000) and when the Aβ concentration rises exponentially, the activation level of the AMPK remained similar to that detected in younger J20 mice (roughly a ~150 percent activation) (Fig. 4F, G). This result indicates that the AMPK activation observed does not depend on age or Aβ in vivo.
Finally, the authors did not address whether the AMPK pathway activated in vivo was modulating the metabolism of APP/Aβ, as suggested earlier by the Marambaud group (Vingtdeux et al., 2010; Vingtdeux et al., 2011).
In summary, I'm not convinced that the presence of oligomeric Aβ assemblies was established in vivo in these animals, or that they cause activation of AMPK (this is particularly important to establish in order to draw a parallel with the studies done in vitro). While APP and its C-terminal fragments may be responsible for the activation of AMPK-CaMKK2 in vivo, that does not appear to be age dependent—a cardinal feature of Alzheimer's disease.
State of aggregation.
Neuron. 1995 Jun;14(6):1105-16.
We would like to thank Dr. Sylvain Lesne for his interest in our work and for his comments. We would like to clarify some of the points he raised concerning our report, since we disagree with some of his statements.
Regarding Aβ detection, we used the 6E10 antibody to monitor the level of APP transgene expression at the age we performed our analysis of spine density in vivo (i.e., three months postnatal), but our intention was not to characterize the presence of Aβ1-42 oligomers at this age in this mouse strain. They have been extensively characterized previously by others (see below). The 6E10 antibody is known to recognize human APP, CTF-β, and Aβ. We acknowledge that the band running below 15 kDa should be considered as a mixture of CTF/Aβ since tissues were homogenized in RIPA buffer. Regarding the APP signal at ~100 kDa, we could clearly detect a faint signal in the wild-type mice (after overexposure of the membrane), suggesting that the antibody can cross-react with mouse APP. This is further supported by the detection of endogenous CTF/Aβ in wild-type control animals with this antibody (Fig. 4B).
Dr. Lesne suggests that liquid phase assays are essential to prove the presence of oligomeric Aβ. The composition of Aβ oligomeric forms in the J20 mouse line was characterized previously by Dr. Lennart Mucke’s group (Cheng et al., 2007; Meilandt et al., 2009). In fact, Dr. Lesne is a coauthor on one of these two papers. Mucke's group showed that, in addition to CTF-β fragments, these mice present high levels of Aβ*56 at three to four months, Aβ1-42 dimers were detected from 17 months, and Aβ1-42 trimers were assessed in five- to six-month-old animals.
We agree that AMPK activation does not seem to correlate with increased Aβ levels in vivo, but this doesn’t mean that Aβ does not activate AMPK in vivo. The absence of further increase in AMPK activation at later ages could simply reflect a threshold effect where AMPK activation peaks when the Aβ oligomers (or other APP cleaved products) reach a certain concentration. Other important pT172-AMPK changes, for example, in localization at the synapse, could go undetected using this simple Western blot approach.
We cautiously mentioned in our report that inhibiting the CAMKK2-AMPK pathway provides protective effects from spine loss in the J20 mouse line at an early age (three to four months), when loss of synapses are detected but prior to the appearance of plaques. However, we acknowledge that further experiments will be needed to determine if inhibiting this pathway is relevant at later stages of the disease in this hAPP-J20 mouse model as well as in other AD mouse models.
We did not address the question of whether the AMPK pathway activated in vivo was modulating the metabolism of APP/Aβ, since this was not the main point of our study. Recently, several reports, besides our own, indicated that Aβ42 oligomers activate AMPK in vitro (Yoon et al., 2013; Son et al, 2011; Thornton et al., 2011). Our goal was to determine if the CAMKK2-AMPK pathway plays a functionally important role in mediating the synaptotoxic effects of Aβ in vivo, paralleling our in-vitro findings, which demonstrated that inhibition of this pathway protects against the synaptotoxic effects of Aβ42 oligomers. Therefore, we employed a strategy to analyze the role of the pathway at the single-cell level and determine if the protection against Aβ42 conferred by blocking the CAMKK2-AMPK pathway is cell autonomous.
The role of AMPK in APP/Aβ metabolism in vivo is definitely another interesting question that implies non-cell-autonomous/paracrine effects on dendritic spine maintenance, and is indeed still controversial. Studies from Marambaud’s group have suggested that AMPK activation might decrease APP cleavage/Aβ production or increase Aβ clearance through autophagy (Vingtdeux et al., 2010; Vingtdeux et al., 2011). On the other hand, other reports indicate that AMPK overactivation increases Aβ generation through β-secretase transcriptional upregulation (Chen et al., 2009) and/or mTOR-dependent protein synthesis inhibition (Yoon et al., 2012). Solving this discrepancy will be interesting and challenging. A definitive answer will be obtained by careful analyses of mouse models of AD in which the catalytic activity of AMPK is genetically deleted. Since there are two isoforms of AMPK-α in mammals, generation of double conditional knockouts for AMPK-α1 and -α2 crossed with an appropriate Cre-driver and the hAPP-J20 transgenic mice would be required (quadruple transgenic mice). This was clearly beyond the scope of our report. However, we agree that this question is interesting, and we (and others) are currently performing some of these experiments.
In summary, the presence of oligomeric forms of Aβ42 as well as other forms of Aβ (Aβ*56 and CTF-Aβ) have been characterized previously in this hAPP-J20 transgenic mouse strain (see Cheng et al., 2007; Meilandt et al., 2009) and are very likely to contribute to AMPK activation in vivo, as we and others have shown in vitro. We combined in-vitro and in-vivo approaches to 1) ascertain the specific effects of Aβ42 oligomers (in vitro), and 2) verify in vivo that in the well-characterized hAPP-J20 model that produces Aβ42 oligomers (Cheng et al., 2007; Meilandt et al., 2009), blocking CAMKK2 or AMPK function protected neurons from the synaptotoxic effects detected at early stages of disease progression. It will indeed be interesting to explore if this CAMKK2-AMPK pathway also mediates the potential synaptotoxic effects of other forms of Aβ (Aβ*56 and CTF-Aβ).
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