The use of human induced pluripotent stem (iPS) and induced neuronal (iN) cells is clearly a major step forward compared to standard clonal cell lines, and also to our own mainstay system, cultured primary AD transgenic neurons. However, the results with induced cells are not yet all that easy to fully interpret. Findings of an elevated Aβ42/40 ratio (Qiang et al., 2011; Yagi et al., 2011) fit well with what is known from prior work on familial AD (FAD) mutations. The enlarged early endosomes seen both by Qiang et al., and now Israel et al., also fit well with pioneering work by Ralph Nixon and Ann Cataldo. Yet it is still unclear if these genetically engineered cells are equivalent to primary human neurons. Israel and colleagues’ evidence that tau is altered by C-terminal fragments (CTFs) of amyloid precursor protein (APP) rather than Aβ builds on a growing literature of Aβ-independent effects due to FAD mutations (here, APP duplications). However, these observations are based on treatment with γ-secretase inhibitors. which can be tricky because of potential effects on iPS cell...  Read more

The use of human induced pluripotent stem (iPS) and induced neuronal (iN) cells is clearly a major step forward compared to standard clonal cell lines, and also to our own mainstay system, cultured primary AD transgenic neurons. However, the results with induced cells are not yet all that easy to fully interpret. Findings of an elevated Aβ42/40 ratio (Qiang et al., 2011; Yagi et al., 2011) fit well with what is known from prior work on familial AD (FAD) mutations. The enlarged early endosomes seen both by Qiang et al., and now Israel et al., also fit well with pioneering work by Ralph Nixon and Ann Cataldo. Yet it is still unclear if these genetically engineered cells are equivalent to primary human neurons. Israel and colleagues’ evidence that tau is altered by C-terminal fragments (CTFs) of amyloid precursor protein (APP) rather than Aβ builds on a growing literature of Aβ-independent effects due to FAD mutations (here, APP duplications). However, these observations are based on treatment with γ-secretase inhibitors. which can be tricky because of potential effects on iPS cell differentiation, toxicity, etc. In addition, a considerable number of divergent data point to Aβ rather than CTFs as being critical.

In our experience, limited (overnight) γ-secretase inhibition did prevent both alterations in synapses (Almeida et al., 2005; Tampellini et al., 2009) and endocytosis (Almeida et al., 2006), albeit in mutant APP-overexpressing transgenic neurons. In contrast, longer treatments (or higher concentrations) with γ-secretase inhibitor were not protective. Based on work in primary transgenic neurons, synapsin 1 would also not be the optimal marker to use for identifying presynaptic changes, which Israel et al. found to be unchanged in their AD iPS cells. It will be important to obtain more data with these exciting new model systems as noted by Abeliovich in the ARF news story.

References:
Qiang L, Fujita R, Yamashita T, Angulo S, Rhinn H, Rhee D, Doege C, Chau L, Aubry L, Vanti WB, Moreno H, Abeliovich A. Directed conversion of Alzheimer's disease patient skin fibroblasts into functional neurons. Cell. 2011 Aug 5;146(3):359-71. Abstract

Yagi T, Ito D, Okada Y, Akamatsu W, Nihei Y, Yoshizaki T, Yamanaka S, Okano H, Suzuki N. Modeling familial Alzheimer's disease with induced pluripotent stem cells. Hum Mol Genet. 2011 Dec 1;20(23):4530-9. Abstract

Almeida CG, Tampellini D, Takahashi RH, Greengard P, Lin MT, Snyder EM, Gouras GK. Beta-amyloid accumulation in APP mutant neurons reduces PSD-95 and GluR1 in synapses. Neurobiol Dis. 2005 Nov;20(2):187-98. Abstract

Tampellini D, Rahman N, Gallo EF, Huang Z, Dumont M, Capetillo-Zarate E, Ma T, Zheng R, Lu B, Nanus DM, Lin MT, Gouras GK. Synaptic activity reduces intraneuronal Abeta, promotes APP transport to synapses, and protects against Abeta-related synaptic alterations. J Neurosci. 2009 Aug 5;29(31):9704-13. Abstract

Almeida CG, Takahashi RH, Gouras GK. Beta-amyloid accumulation impairs multivesicular body sorting by inhibiting the ubiquitin-proteasome system. J Neurosci. 2006 Apr 19;26(16):4277-88. Abstract

View all comments by Gunnar K. Gouras

This paper adds to a growing number of publications using iPSCs to study AD-related cellular phenotypes in vitro (Qiang et al., 2011; Israel et al., 2012; Yagi et al., 2011; Shi et al., 2012). It is reassuring to see so many labs independently finding robust phenotypes in their various cell lines. The fact that these cells converge on similar phenotypes with respect to altered APP processing is interesting, and I think we can now be confident that patient-derived neurons are a good model for AD pathogenesis. The next step is to determine the mechanism(s) by which these observed differences in APP processing are leading to cell death in AD. I hope we'll start to see reports where patient-derived neurons are being used to uncover novel disease mechanisms.

For me, the most interesting aspect of this paper is the differential responsiveness to DHA. Understanding why certain cell lines are responsive to treatments whilst others are not could ultimately have implications in the clinic: The success of a particular treatment could depend on patients being "subtyped" appropriately....  Read more

This paper adds to a growing number of publications using iPSCs to study AD-related cellular phenotypes in vitro (Qiang et al., 2011; Israel et al., 2012; Yagi et al., 2011; Shi et al., 2012). It is reassuring to see so many labs independently finding robust phenotypes in their various cell lines. The fact that these cells converge on similar phenotypes with respect to altered APP processing is interesting, and I think we can now be confident that patient-derived neurons are a good model for AD pathogenesis. The next step is to determine the mechanism(s) by which these observed differences in APP processing are leading to cell death in AD. I hope we'll start to see reports where patient-derived neurons are being used to uncover novel disease mechanisms.

For me, the most interesting aspect of this paper is the differential responsiveness to DHA. Understanding why certain cell lines are responsive to treatments whilst others are not could ultimately have implications in the clinic: The success of a particular treatment could depend on patients being "subtyped" appropriately. However, given that this study only examines cells from two familial patients and two sporadic patients, it is difficult to draw any firm conclusions in that respect without expanding this study to include more patients.

References:
Qiang L, Fujita R, Yamashita T, Angulo S, Rhinn H, Rhee D, Doege C, Chau L, Aubry L, Vanti WB, Moreno H, Abeliovich A. Directed conversion of Alzheimer's disease patient skin fibroblasts into functional neurons. Cell. 2011 Aug 5;146(3):359-71. Abstract

Israel MA, Yuan SH, Bardy C, Reyna SM, Mu Y, Herrera C, Hefferan MP, Van Gorp S, Nazor KL, Boscolo FS, Carson CT, Laurent LC, Marsala M, Gage FH, Remes AM, Koo EH, Goldstein LS. Probing sporadic and familial Alzheimer's disease using induced pluripotent stem cells. Nature. 2012 Feb 9;482(7384):216-20. Abstract

Yagi T, Ito D, Okada Y, Akamatsu W, Nihei Y, Yoshizaki T, Yamanaka S, Okano H, Suzuki N. Modeling familial Alzheimer's disease with induced pluripotent stem cells. Hum Mol Genet. 2011 Dec 1;20(23):4530-9. Abstract

Shi Y, Kirwan P, Smith J, Maclean G, Orkin SH, Livesey FJ. A human stem cell model of early Alzheimer's disease pathology in Down syndrome. Sci Transl Med. 2012 Mar 7;4(124):124ra29. Abstract

View all comments by Selina Wray

Marcello and colleagues demonstrate that synaptic activity regulates the levels of ADAM10 on the plasma membrane. Long-term potentiation (LTP) induces ADAM10 internalization by clathrin-mediated endocytosis (CME), whereas LTD induces its insertion into the plasma membrane. There is a long history of literature linking CME and Aβ; however, those studies generally revolve around APP internalization and Aβ generation. Work from our group and others shows that synaptic activity causes CME of APP, which increases Aβ production in endosomes. The data in the Marcello paper look at this from a different angle. Here, synaptic activity increases ADAM10 internalization, which decreases its ability to cleave APP; in theory, this would then increase Aβ generation. So taken together, this suggests that synaptic activity and CME may promote Aβ generation by two parallel pathways: 1) increasing amyloidogenic processing of APP within endosomes, and 2) decreasing non-amyloidogenic processing of APP at the plasma membrane.

As with any good study, lots of questions remain. Is ADAM10...  Read more

Marcello and colleagues demonstrate that synaptic activity regulates the levels of ADAM10 on the plasma membrane. Long-term potentiation (LTP) induces ADAM10 internalization by clathrin-mediated endocytosis (CME), whereas LTD induces its insertion into the plasma membrane. There is a long history of literature linking CME and Aβ; however, those studies generally revolve around APP internalization and Aβ generation. Work from our group and others shows that synaptic activity causes CME of APP, which increases Aβ production in endosomes. The data in the Marcello paper look at this from a different angle. Here, synaptic activity increases ADAM10 internalization, which decreases its ability to cleave APP; in theory, this would then increase Aβ generation. So taken together, this suggests that synaptic activity and CME may promote Aβ generation by two parallel pathways: 1) increasing amyloidogenic processing of APP within endosomes, and 2) decreasing non-amyloidogenic processing of APP at the plasma membrane.

As with any good study, lots of questions remain. Is ADAM10 internalized into separate endosomes than APP? Or can ADAM10 and APP still be within the same endosome but maybe the low pH prevents further cleavage? How is ADAM10 trafficked or segregated after CME? The authors demonstrate that activity modulates APP cleavage by ADAM10, which is consistent with a reduction in Aβ, but one thing lacking in this paper is a direct measurement that Aβ generation is changing as a consequence of the altered ADAM10 subcellular localization.

View all comments by John Cirrito

A disintegrin and metalloproteinase 10 (ADAM10) is apparently one of the most critical membrane-associated proteases in the central nervous system (CNS). Its prominent role in the embryonic and adult CNS has been revealed by a number of studies. Next to APP, an increasing number of transmembrane proteins, including Notch receptors and ligands, are subject to ADAM10-mediated shedding. These shedding events are of critical importance to modulate postsynaptic function and synaptic plasticity.

Based on their previous work, Monica Di Luca´s group convincingly addressed the post-transcriptional regulation of ADAM10 in neurons. Both its transport to the postsynaptic membrane and its removal are central events to regulate synaptic functions, morphology, and the processing of important substrates, including APP. In the current study, the authors focus on the endocytosis of ADAM10 from the postsynaptic membrane. Using mainly coimmunoprecipitation experiments, they showed that, like other surface molecules, ADAM10 endocytosis depends on binding to the clathrin adaptor AP2. This binding...  Read more

A disintegrin and metalloproteinase 10 (ADAM10) is apparently one of the most critical membrane-associated proteases in the central nervous system (CNS). Its prominent role in the embryonic and adult CNS has been revealed by a number of studies. Next to APP, an increasing number of transmembrane proteins, including Notch receptors and ligands, are subject to ADAM10-mediated shedding. These shedding events are of critical importance to modulate postsynaptic function and synaptic plasticity.

Based on their previous work, Monica Di Luca´s group convincingly addressed the post-transcriptional regulation of ADAM10 in neurons. Both its transport to the postsynaptic membrane and its removal are central events to regulate synaptic functions, morphology, and the processing of important substrates, including APP. In the current study, the authors focus on the endocytosis of ADAM10 from the postsynaptic membrane. Using mainly coimmunoprecipitation experiments, they showed that, like other surface molecules, ADAM10 endocytosis depends on binding to the clathrin adaptor AP2. This binding seems to be stronger in samples from AD patients. It was shown that abolishing this binding using mutants or pharmacological approaches also reduced ADAM10 endocytosis. Interestingly, the authors found that long-term potentiation induced this process, whereas long-term depression had an opposite effect by stimulating interaction of ADAM10 and SAP97, thereby promoting ADAM10 delivery to the plasma membrane. In a last set of experiments, they showed that this dynamic, and apparently ADAM10-dependent regulation, also led to a differential processing of APP at the α-secretase site. ADAM10 localization at the synaptic membrane more or less determines whether α-secretase processing occurs.

What are the consequences for AD? The findings certainly increase our basic understanding of how APP processing, synaptic remodeling, and cellular localization of ADAM10 are linked. I do not feel that immediate new therapeutic targets are apparent, since the factors involved affect a number of other proteins as well; for example, the fine-tuned localization of ADAM10 is necessary for the degree of shedding of most likely more than 10 other synaptic membrane proteins. Also, AP2 mediates endocytosis of a huge number of surface proteins. On the other hand, additional intracellular and extracellular factors are needed to control the activity of ADAM10. We and others showed recently, for example, that the integration of ADAM10 in the tetraspanin web is instrumental for forward trafficking of the protease. It is likely that additional factors directly contribute to the cellular localization of ADAM10.

As discussed by the authors, the Aβ levels in vivo are in part dependent on the activity of ADAM10. This study additionally provides evidence that the degree of neuronal and synaptic activity alters the function (and localization) of ADAM10 and the degree of Aβ production. Based on their initial observations that in AD brains, the endocytic route of ADAM10 is favored, this would directly explain both the increased Aβ production and the problems in synaptogenes as reported in AD.

View all comments by Paul Saftig

I read this paper by Marcello et al. with much interest, and I was impressed both by the number of data and the coherence of the hypothesis. It explains the role played by neuronal activity in Aβ secretion. It also sheds new light on the connection between endocytosis and Aβ.

In addition, it opens new research perspectives: The alteration of ADAM10/AP2 association in AD is currently not explained and could be related to changes in the cell membrane itself. We have, in this respect, shown that increases in membrane cholesterol favor endocytosis and production of Aβ (see Marquer et al., 2011; Cossec et al., 2010).

I'd add a word of caution on the neuropathology. Only six cases were examined at Braak stage IV—these were apparently the same cases the authors studied before (see Marcello et al., 2012). Braak stage IV pathology is common in asymptomatic aged persons. The diagnostic probability of Alzheimer's disease is only ranked as intermediate in the current diagnostic criteria (Hyman et al., 2012; Montine et al., 2012). Such cases are, by definition, free from tau pathology...  Read more

I read this paper by Marcello et al. with much interest, and I was impressed both by the number of data and the coherence of the hypothesis. It explains the role played by neuronal activity in Aβ secretion. It also sheds new light on the connection between endocytosis and Aβ.

In addition, it opens new research perspectives: The alteration of ADAM10/AP2 association in AD is currently not explained and could be related to changes in the cell membrane itself. We have, in this respect, shown that increases in membrane cholesterol favor endocytosis and production of Aβ (see Marquer et al., 2011; Cossec et al., 2010).

I'd add a word of caution on the neuropathology. Only six cases were examined at Braak stage IV—these were apparently the same cases the authors studied before (see Marcello et al., 2012). Braak stage IV pathology is common in asymptomatic aged persons. The diagnostic probability of Alzheimer's disease is only ranked as intermediate in the current diagnostic criteria (Hyman et al., 2012; Montine et al., 2012). Such cases are, by definition, free from tau pathology in the neocortex. Usually, Aβ deposits are, however, already present. This could be determined in the studied cases, for instance, by identifying the stage of amyloid pathology (see Thal et al., 2002). It would be interesting to compare the association of ADAM10/AP2 in the hippocampus (with tangle pathology) and in the frontal cortex (devoid of tangle pathology but possibly with Aβ deposition). Tau accumulation, which may affect synapses, could play a central role in the alteration of ADAM10/AP2 association, as it probably does for the clathrin adaptor PICALM, which we found to colocalize with tangles (Ando et al., 2013). New studies with more advanced cases would strengthen the human data.

References:
Marquer C, Devauges V, Cossec JC, Liot G, Lécart S, Saudou F, Duyckaerts C, Lévêque-Fort S, Potier MC. Local cholesterol increase triggers amyloid precursor protein-Bace1 clustering in lipid rafts and rapid endocytosis. FASEB J. 2011 Apr;25(4):1295-305. Abstract

Cossec JC, Simon A, Marquer C, Moldrich RX, Leterrier C, Rossier J, Duyckaerts C, Lenkei Z, Potier MC. Clathrin-dependent APP endocytosis and Aβ secretion are highly sensitive to the level of plasma membrane cholesterol. Biochim Biophys Acta. 2010 Aug;1801(8):846-52. Abstract

Marcello E, Epis R, Saraceno C, Gardoni F, Borroni B, Cattabeni F, Padovani A, Di Luca M. SAP97-mediated local trafficking is altered in Alzheimer disease patients' hippocampus. Neurobiol Aging. 2012 Feb;33(2):422.e1-10. Abstract

Hyman BT, Phelps CH, Beach TG, Bigio EH, Cairns NJ, Carrillo MC, Dickson DW, Duyckaerts C, Frosch MP, Masliah E, Mirra SS, Nelson PT, Schneider JA, Thal DR, Thies B, Trojanowski JQ, Vinters HV, Montine TJ. National Institute on Aging-Alzheimer's Association guidelines for the neuropathologic assessment of Alzheimer's disease. Alzheimers Dement. 2012 Jan;8(1):1-13. Abstract

Montine TJ, Phelps CH, Beach TG, Bigio EH, Cairns NJ, Dickson DW, Duyckaerts C, Frosch MP, Masliah E, Mirra SS, Nelson PT, Schneider JA, Thal DR, Trojanowski JQ, Vinters HV, Hyman BT, National Institute on Aging, Alzheimer’s Association. National Institute on Aging-Alzheimer's Association guidelines for the neuropathologic assessment of Alzheimer's disease: a practical approach. Acta Neuropathol. 2012 Jan;123(1):1-11. Abstract

Thal DR, Rüb U, Orantes M, Braak H. Phases of A β-deposition in the human brain and its relevance for the development of AD. Neurology. 2002 Jun 25;58(12):1791-800. Abstract

Ando K, Brion JP, Stygelbout V, Suain V, Authelet M, Dedecker R, Chanut A, Lacor P, Lavaur J, Sazdovitch V, Rogaeva E, Potier MC, Duyckaerts C. Clathrin adaptor CALM/PICALM is associated with neurofibrillary tangles and is cleaved in Alzheimer's brains. Acta Neuropathol. 2013 Apr 16. Abstract

View all comments by Charles Duyckaerts