An amazing technology used to show, once and for all, that plaques are dynamic structures. A great paper which subverts the huge literature seeking to correlate plaque numbers with clinical features. Pathology does not wait around to be counted!!!"

View all comments by John Hardy

This is another state-of-the-art paper by the group active on in vivo two-photon imaging on mouse models of amyloidogenesis, and it gives important clues for Alzheimer disease pathogenesis. The paper shows, for the first time, that dense plaques in mouse models reach their maximum size in about a day and thereafter maintain a status quo. This does not follow the simple, size-dependent law of mass action, as even small plaques do not grow any further. This is a most amazing finding and abrogates all prior preconceived notions that plaques grow slowly over life and that given time, all plaques would reach a maximum size. Importantly, the quick growth of dense plaques suggests that dense plaques grow not only with Aβ monomer addition, but perhaps also by capturing oligomeric intermediates at the fiber ends, as shown earlier for prion proteins (Serio et al., 2000; Collins et al., 2004).

Why dense plaques stop growing suddenly is just as intriguing. Quick recruitment of macrophages at sites of dense plaque formation, as shown here, could be one mechanism, but the provided images...  Read more

This is another state-of-the-art paper by the group active on in vivo two-photon imaging on mouse models of amyloidogenesis, and it gives important clues for Alzheimer disease pathogenesis. The paper shows, for the first time, that dense plaques in mouse models reach their maximum size in about a day and thereafter maintain a status quo. This does not follow the simple, size-dependent law of mass action, as even small plaques do not grow any further. This is a most amazing finding and abrogates all prior preconceived notions that plaques grow slowly over life and that given time, all plaques would reach a maximum size. Importantly, the quick growth of dense plaques suggests that dense plaques grow not only with Aβ monomer addition, but perhaps also by capturing oligomeric intermediates at the fiber ends, as shown earlier for prion proteins (Serio et al., 2000; Collins et al., 2004).

Why dense plaques stop growing suddenly is just as intriguing. Quick recruitment of macrophages at sites of dense plaque formation, as shown here, could be one mechanism, but the provided images do not show a complete walling off. This suggests that other, less simple mechanisms are at play, including local Aβ production and trafficking.

A recent, interesting study in a mouse model of amyloidogenesis showed that FAD APP mutations cause axonal trafficking defects and that that, in turn, stimulates the proteolytic processing of APP and generation of Aβ (Stokin et al., 2005). This fit well with development of dystrophic neurites (DNs) in these mice, preceding plaques by more than a year (Stokin et al., 2005). Meyer-Luehmann and colleagues, studying the temporal relation between rapidly growing plaques and DNs, showed that although DNs were observed in plaque-free areas, DNs were more pronounced near plaques. Conversely, DNs in the plaque-free areas did not seem to cause dense plaques, and mice did not deposit diffuse plaques at this age. It would be worth studying this in more detail over a shorter time window, as DNs were also observed to change morphologies and even resolve, leaving open the possibility that diffuse or pre-diffuse plaques are formed but are quickly turned over. Moreover, the current resolution of this technique does not permit visualization of small, dense plaques, and at times it leaves the reader guessing whether some of the punctate blue staining might potentially be “sub-microscopic” dense plaques (for instance, boxed areas in Figure 1). Also, sometimes the capillary network is strangely absent, for instance, some panels of Figure 1 representing APPswe/PS1d9xYFP mice, where the plaque-vessel relationship has been alluded to on a small sample size.

Clearly, more work is needed before we can have all the answers. Meanwhile, despite some of its limitations, in vivo multiphoton imaging remains a valuable technique. We hope to see more results coming out from this technology, especially with higher objectives, use of confocal settings, and perhaps a shorter time lapse, even though that would make it even more labor-intensive. Parallel detailed histological analysis including ultrastructural microscopy should make it even more interesting.

References:
Collins SR, Douglass A, Vale RD, Weissman JS. Mechanism of prion propagation: amyloid growth occurs by monomer addition. PLoS Biol. 2004 Oct 1;2(10):e321. Abstract

Meyer-Luehmann M, Spires-Jones TL, Prada C, Garcia-Alloza M, de Calignon A, Rozkalne A, Koenigsknecht-Talboo J, Holtzman DM, Bacskai BJ, Hyman BT. Rapid appearance and local toxicity of amyloid-β plaques in a mouse model of Alzheimer's disease. Nature. 2008 Feb 7;451(7179):720-4. Abstract

Serio TR, Cashikar AG, Kowal AS, Sawicki GJ, Moslehi JJ, Serpell L, Arnsdorf MF, Lindquist SL. Nucleated conformational conversion and the replication of conformational information by a prion determinant. Science. 2000 Aug 25;289(5483):1317-21. Abstract

Spires TL, Meyer-Luehmann M, Stern EA, McLean PJ, Skoch J, Nguyen PT, Bacskai BJ, Hyman BT. Dendritic spine abnormalities in amyloid precursor protein transgenic mice demonstrated by gene transfer and intravital multiphoton microscopy. J Neurosci. 2005 Aug 3;25(31):7278-87. Abstract

Stokin GB, Lillo C, Falzone TL, Brusch RG, Rockenstein E, Mount SL, Raman R, Davies P, Masliah E, Williams DS, Goldstein LS. Axonopathy and transport deficits early in the pathogenesis of Alzheimer's disease. Science. 2005 Feb 25;307(5713):1282-8. Abstract

View all comments by Samir Kumar-Singh

Meyer-Luehmann et al. provide a spectacular and informative high-tech view of the kinetics of amyloid formation and its potential consequences in a mouse model of Alzheimer disease. Although some may regard their findings as contradictory to the idea that early transport defects may play a role in neuronal dysfunction and in the enhancement of amyloid formation in Alzheimer disease, I do not see this study as being in conflict with those ideas. There are two major points:

1. In addition to the experiments we reported (Stokin et al., 2005), there are a number of previous studies (cited in Stokin et al., 2005) that find significant axonal dystrophies, which may be indicative of defects in axonal transport, prior to amyloid deposition as well as in regions of the brain that lack amyloid deposition. A related issue is that the experiments of Meyer-Luehmann et al. primarily focus on cortical regions, while many of the experiments in Stokin et al. examined basal forebrain cholinergic axons, which are long projection axons in regions...  Read more

Meyer-Luehmann et al. provide a spectacular and informative high-tech view of the kinetics of amyloid formation and its potential consequences in a mouse model of Alzheimer disease. Although some may regard their findings as contradictory to the idea that early transport defects may play a role in neuronal dysfunction and in the enhancement of amyloid formation in Alzheimer disease, I do not see this study as being in conflict with those ideas. There are two major points:

1. In addition to the experiments we reported (Stokin et al., 2005), there are a number of previous studies (cited in Stokin et al., 2005) that find significant axonal dystrophies, which may be indicative of defects in axonal transport, prior to amyloid deposition as well as in regions of the brain that lack amyloid deposition. A related issue is that the experiments of Meyer-Luehmann et al. primarily focus on cortical regions, while many of the experiments in Stokin et al. examined basal forebrain cholinergic axons, which are long projection axons in regions distinct from the cortical regions imaged in the present paper.

2. A number of papers (Torroja et al., 1999; Gunawardena et al., 2001; Salehi et al., 2006; Pigino et al., 2003) report that APP overexpression and presenilin mutations, both of which can cause Alzheimer disease, can induce serious abnormalities in axonal transport in the absence of plaque formation or human Aβ. Again, these findings and those of Meyer-Luehmann et al. are not in conflict and may in some sense be complementary.

A possibly useful way to think about this collection of observations is by asking two questions: 1) Are amyloid plaques completely benign? 2) Are amyloid plaques or their constituents sufficient to induce all of the biochemical and cellular malfunctions associated with Alzheimer disease? The answer to the first question is almost surely no—it is hard to imagine that massive numbers of amyloid plaques in the brain would not interfere with neuronal function. Indeed, there is ample documentary evidence that amyloid constituents can be neurotoxic to varying degrees. The answer to the second question is far from clear. APP processing clearly generates fragments other than Aβ, and may influence other biological activities of APP and neurons. It may be that a productive additional approach to this question is by trying to understand the relative toxicity of Aβ to neurons compared to toxicity caused by other environmental influences, mutations, pathways, or polymorphisms that all may reduce or impair the supply of critical materials to synapses by the axonal transport machinery.

View all comments by Larry Goldstein

This is a beautiful paper showing plaque growth in vivo. I cordially disagree on one point: the authors state that the speed of an individual plaque's growth is surprising because of prior in vitro studies of protein aggregation showing a slow, time-dependent course. The discussion appears to suggest that the appearance of a plaque within a day or two does not fit in with data on nucleation-dependent polymerization (Jarrett and Lansbury, 1993). In fact, the observations in this paper are reminiscent of seeded crystal growth. Live multiphoton imaging cannot yet visualize the prior accumulation of Aβ or the nucleation event, but once nucleation happens, growth should be very fast. The rate of growth measured in this study is exactly what the nucleation model would predict. It is gratifying to see in vivo.

View all comments by Peter Lansbury

This paper is intriguing, to say the least. The authors succeeded in monitoring in vivo the formation of dense-core plaques. Surprisingly, they observed that across different mouse models of Alzheimer disease, plaques formed quite rapidly (24 hours) but rarely. One of the most interesting observations of the paper is the temporal relation between rapid dense-core plaque appearance, microglial recruitment, and neuritic changes. Morphological changes of neurites never preceded plaque appearance and/or microglia migration towards the site of the newly formed plaque. Microglia did not seem to either facilitate or clear plaques, suggesting that they may participate in stabilizing plaque size after their initial acute growth.

If we consider that, in recent years, soluble Aβ oligomers rather than Aβ fibrils in plaques have come to be seen as the “real bad guys” (Walsh and Selkoe, 2007), these new findings raise nearly as many questions as they answer. For example, are the neuritic alterations described in this paper induced by soluble forms of amyloid-β or by plaques themselves? If...  Read more

This paper is intriguing, to say the least. The authors succeeded in monitoring in vivo the formation of dense-core plaques. Surprisingly, they observed that across different mouse models of Alzheimer disease, plaques formed quite rapidly (24 hours) but rarely. One of the most interesting observations of the paper is the temporal relation between rapid dense-core plaque appearance, microglial recruitment, and neuritic changes. Morphological changes of neurites never preceded plaque appearance and/or microglia migration towards the site of the newly formed plaque. Microglia did not seem to either facilitate or clear plaques, suggesting that they may participate in stabilizing plaque size after their initial acute growth.

If we consider that, in recent years, soluble Aβ oligomers rather than Aβ fibrils in plaques have come to be seen as the “real bad guys” (Walsh and Selkoe, 2007), these new findings raise nearly as many questions as they answer. For example, are the neuritic alterations described in this paper induced by soluble forms of amyloid-β or by plaques themselves? If the former, why would soluble Aβ oligomers be responsible for dysmorphic neurites only when released by the plaques, but not as precursors of the same plaques? And even if we assume that these newly formed plaques function just as a local source of highly concentrated soluble Aβ, then why would dystrophic neurites take several days to appear? Faster morphological, functional, and behavioral alterations induced by soluble Aβ oligomers have been described both in cultured neurons (Calabrese et al., 2007), slices (Klyubin et al., 2005; Shankar et al., 2007), and in living rats (Cleary et al., 2005).

To prove a direct role for plaques in inducing morphological changes, one would have to show that a selectively induced disassembly of the dense-core plaques would result in microglia dispersion and restoration of normal neurites. Recently, Martins et al. (2008) have shown that biologically relevant lipids, including lipid extracts from brain, can revert inert Aβ amyloid fibrils into neurotoxic protofibrils. Microglia could use a similar mechanism to control the size of amyloid-β plaques.

Finally, the biggest challenge will be to identify the predictors for such sudden and local appearance of amyloid-β plaques so that eventually their formation can be prevented.

References:
Calabrese B, Shaked GM, Tabarean IV, Braga J, Koo EH, Halpain S. Rapid, concurrent alterations in pre- and postsynaptic structure induced by naturally-secreted amyloid-beta protein. Mol Cell Neurosci. 2007 Jun 1;35(2):183-93. Abstract

Cleary JP, Walsh DM, Hofmeister JJ, Shankar GM, Kuskowski MA, Selkoe DJ, Ashe KH. Natural oligomers of the amyloid-beta protein specifically disrupt cognitive function. Nat Neurosci. 2005 Jan 1;8(1):79-84. Abstract

Klyubin I, Walsh DM, Lemere CA, Cullen WK, Shankar GM, Betts V, Spooner ET, Jiang L, Anwyl R, Selkoe DJ, Rowan MJ. Amyloid beta protein immunotherapy neutralizes Abeta oligomers that disrupt synaptic plasticity in vivo. Nat Med. 2005 May 1;11(5):556-61. Abstract

Martins IC, Kuperstein I, Wilkinson H, Maes E, Vanbrabant M, Jonckheere W, Van Gelder P, Hartmann D, D'Hooge R, De Strooper B, Schymkowitz J, Rousseau F. Lipids revert inert Abeta amyloid fibrils to neurotoxic protofibrils that affect learning in mice. EMBO J. 2008 Jan 9;27(1):224-33. Abstract

Shankar GM, Bloodgood BL, Townsend M, Walsh DM, Selkoe DJ, Sabatini BL. Natural oligomers of the Alzheimer amyloid-beta protein induce reversible synapse loss by modulating an NMDA-type glutamate receptor-dependent signaling pathway. J Neurosci. 2007 Mar 14;27(11):2866-75. Abstract

Walsh DM, Selkoe DJ. A beta oligomers - a decade of discovery. J Neurochem. 2007 Jun 1;101(5):1172-84. Abstract

View all comments by Barbara Calabrese

I agree with Peter Lansbury. This is a beautiful piece of research, though the rapidity with which distinct plaques could be visualized is not surprising to those of us who have studied the deposition of beta-amyloid in near-physiological milieu in vitro. Neither should we be surprised that plaque formation was coincident with both an immune/inflammatory response and damage to the environment adjacent to the plaque.

I picked up my electronic copy of Nature early on the 7th and was able to present these results to medical students at 10.00 a.m. that morning. Congratulations are due to the authors for making simple what must have been extremely difficult to achieve.

View all comments by Chris Exley

This captures the extraordinarily rapid growth of Aβ plaques in real time using sophisticated longitudinal multiphoton microscopy. Microglial cells are “caught in the act” of activation and recruitment. The data make for compelling watching, almost like witnessing a crime.

What does this data suggest in terms of AD patients being treated with drugs to lower Aβ and to inhibit plaque formation? Do these new findings suggest that if drug treatment is discontinued, plaque growth and neurite dystrophy would recommence within days?

In addition, this study reminds me of a previous finding, incredible though it seemed at the time, that AD model mice demonstrated immediate cognitive improvement after passive anti-Aβ immunization (Dodart et al., 2002; Kotilinek et al., 2002). It would be informative if the in-vivo microscopy could be used after immunization to observe microglial activation and recruitment, since the technique has already been used to monitor neurite dystrophy following...  Read more

This captures the extraordinarily rapid growth of Aβ plaques in real time using sophisticated longitudinal multiphoton microscopy. Microglial cells are “caught in the act” of activation and recruitment. The data make for compelling watching, almost like witnessing a crime.

What does this data suggest in terms of AD patients being treated with drugs to lower Aβ and to inhibit plaque formation? Do these new findings suggest that if drug treatment is discontinued, plaque growth and neurite dystrophy would recommence within days?

In addition, this study reminds me of a previous finding, incredible though it seemed at the time, that AD model mice demonstrated immediate cognitive improvement after passive anti-Aβ immunization (Dodart et al., 2002; Kotilinek et al., 2002). It would be informative if the in-vivo microscopy could be used after immunization to observe microglial activation and recruitment, since the technique has already been used to monitor neurite dystrophy following passive (or active) immunization (Brendza et al., 2005; Lombardo et al., 2003). The technique could be very useful to determine the relationship between microglial activation and neurite dystrophy in response to changing levels of plaque Aβ.

View all comments by Gwendolyn Wong

Using sophisticated life imaging techniques, Meyer-Luehmann and colleagues looked at plaque formation in the mouse brain in real time. The paper shows exciting results demonstrating that plaque deposition is very fast and that many of the pathological changes associated with plaques do not precede, but follow deposition, suggesting a cause-consequence relationship. The overall picture emerging is that plaques rapidly crystallize out of solution. Obviously, as interesting as it is, this work does not address the question of the mechanism of toxicity, neither of what determines the dynamics and the rapid precipitation of plaques in the brain.

In our hands amyloid fibrils, as they are supposed to be present in amyloid plaques, display very little toxicity as such. Only when these mature fibrils become resolubilized, for instance, by lipids, do we generate what we called backward oligomers, which exert severe toxicity in neuronal culture and in brain of living animals (Martins and Kuperstein et al., 2008). There is also a recent study by Lesne and colleagues (2008) in...  Read more

Using sophisticated life imaging techniques, Meyer-Luehmann and colleagues looked at plaque formation in the mouse brain in real time. The paper shows exciting results demonstrating that plaque deposition is very fast and that many of the pathological changes associated with plaques do not precede, but follow deposition, suggesting a cause-consequence relationship. The overall picture emerging is that plaques rapidly crystallize out of solution. Obviously, as interesting as it is, this work does not address the question of the mechanism of toxicity, neither of what determines the dynamics and the rapid precipitation of plaques in the brain.

In our hands amyloid fibrils, as they are supposed to be present in amyloid plaques, display very little toxicity as such. Only when these mature fibrils become resolubilized, for instance, by lipids, do we generate what we called backward oligomers, which exert severe toxicity in neuronal culture and in brain of living animals (Martins and Kuperstein et al., 2008). There is also a recent study by Lesne and colleagues (2008) in plaque-bearing aged mice showing that memory impairment depends on soluble oligomers and not on the plaques. Thus, we remain with the question of whether the precipitated plaques seen by Meyer-Luehmann and colleagues directly cause toxicity, or whether around these plaques soluble oligomers or protofibrils are generated in dynamic equilibrium with the suddenly appearing plaques. If such an equilibrium exists, one would, however, have anticipated that plaques would appear and disappear, while these authors find that they are rather stable once they are formed. On the other hand, this exchange between soluble species and plaques may involve a limited number of deposited fibrils in the plaques that possibly does not change the structure of the plaque core once deposited, but is sufficient to produce toxic backward oligomers. Therefore, follow-up study of the presence of soluble oligomers, and of the dynamics of their appearance around the plaque, would be very interesting.

One of the main questions raised by several commentators is, If soluble oligomers are the major source of neurotoxicity, then why does neurodystrophy appear after the deposition of amyloid and not before? Conclusions cannot be made in this regard. There is the whole problem of kinetics and concentration-dependent factors that have to be taken into account. It could be that the threshold for toxicity is close to the threshold for amyloid precipitation, and that only close to the plaques sufficient oligomers are present to cause toxicity. The final neurodystrophy effect seen 5 days after the plaque “birthday” could also be simply the result of a relatively slow cumulative effect of protofibrils/oligomers present before deposition and maintained afterwards by “backward” toxic species released from this newborn plaque.

The question about stability of plaques as a function of time is also very intriguing. It would be informative to compare stability of fibrils in old and newborn plaques: the current study shows only the net result. In addition, it could indeed be that the microglia control the plaque size (Herber et al., 2007). In this regard, it becomes an interesting question to ask whether microglia are then protective, ensuring clearance of soluble toxic species, or whether they accelerate (backward) oligomer release.

References:
Martins IC, Kuperstein I, Wilkinson H, Maes E, Vanbrabant M, Jonckheere W, Van Gelder P, Hartmann D, D'Hooge R, De Strooper B, Schymkowitz J, Rousseau F. Lipids revert inert Abeta amyloid fibrils to neurotoxic protofibrils that affect learning in mice. EMBO J. 2008 Jan 9;27(1):224-33. Abstract

Lesné S, Kotilinek L, Ashe KH. Plaque-bearing mice with reduced levels of oligomeric amyloid-beta assemblies have intact memory function. Neuroscience. 2008 Feb 6 ; 151(3):745-9. Abstract

Herber DL, Mercer M, Roth LM, Symmonds K, Maloney J, Wilson N, Freeman MJ, Morgan D, Gordon MN. Microglial activation is required for Abeta clearance after intracranial injection of lipopolysaccharide in APP transgenic mice. J Neuroimmune Pharmacol. 2007 Jun;2(2):222-31. Epub 2007 Mar 27. Abstract

View all comments by Inna Kuperstein
View all comments by Ivo Martins

This is an excellent study by Lee and colleagues employing serial multiphoton microscopy (MPM) to provide more clues to the process of plaque formation in the living brain of a well-established APP/PS1 transgenic mouse model of Alzheimer disease. Previous work by Hyman and colleagues had provided novel observations on the remarkably rapid appearance of plaques, and had also noted that once formed, there was little additional growth in the size of plaques. The focus of the current study is less the appearance and more the growth in the size of existing plaques over a time frame of a few weeks using a thinned skull window approach. They provide intriguing evidence for the importance of the type of window used to visualize plaques. Specifically, Lee and colleagues show that the open craniotomy with coverslip approach used in previous MPM studies in AD prevents the further growth of plaques and even augments regression of some plaques when compared with the thin-skull method. With the open- but not thin-skull method, there is marked cortical activation of inflammatory cells below the...  Read more

This is an excellent study by Lee and colleagues employing serial multiphoton microscopy (MPM) to provide more clues to the process of plaque formation in the living brain of a well-established APP/PS1 transgenic mouse model of Alzheimer disease. Previous work by Hyman and colleagues had provided novel observations on the remarkably rapid appearance of plaques, and had also noted that once formed, there was little additional growth in the size of plaques. The focus of the current study is less the appearance and more the growth in the size of existing plaques over a time frame of a few weeks using a thinned skull window approach. They provide intriguing evidence for the importance of the type of window used to visualize plaques. Specifically, Lee and colleagues show that the open craniotomy with coverslip approach used in previous MPM studies in AD prevents the further growth of plaques and even augments regression of some plaques when compared with the thin-skull method. With the open- but not thin-skull method, there is marked cortical activation of inflammatory cells below the cranial window. These data also provide further evidence for the importance of inflammatory cells in modulating plaque pathology. One wonders whether the different methods have a differential effect on neuritic dystrophy. Interestingly, they also show that in younger but not older mice, γ-secretase inhibition retards formation and growth of new plaques while not effecting existing plaques. They suggest that these data support the importance of early rather than later therapeutic intervention in AD, although one can note that AD is also an anatomically progressive disease; less vulnerable brain regions may be at an earlier pathological stage (and therefore more amenable to treatment) than more vulnerable/pathologically advanced brain regions. Additionally, they show that the growth of plaques correlates with extracellular β amyloid levels in the interstitial fluid, a pool of β amyloid that many but not all view as the origin of plaques. Overall, this new MPM study is another important contribution in elucidating the development of β amyloid plaque pathology.

View all comments by Gunnar K. Gouras

Yan and colleagues add another piece to the plaque kinetics puzzle by showing, with on multiphoton in vivo microscopy, that amyloid plaques in a bigenic PSAPP mouse model appear and grow over a period of weeks before reaching a mature size. These data seem to be in apparent conflict with earlier work using the same technique on related mouse models (Meyer-Luehmann et al. 2008), where dense plaques were shown to reach their maximum size in about a day and thereafter maintain a status quo.

The present study also goes forward to propose a reason for this discrepancy. Amyloid imaging through large open-skull cranial windows (as utilized solely by Meyer-Luehmann and colleagues) seems to activate gliosis, in contrast to thinned-skull windows of ≈1/10th the size, where calvaria are merely thinned down to allow in vivo microscopy without exposing the dura mater. This seems logical, as activation of gliosis has been shown in several studies to be an important factor in limiting plaque growth (  Read more

Yan and colleagues add another piece to the plaque kinetics puzzle by showing, with on multiphoton in vivo microscopy, that amyloid plaques in a bigenic PSAPP mouse model appear and grow over a period of weeks before reaching a mature size. These data seem to be in apparent conflict with earlier work using the same technique on related mouse models (Meyer-Luehmann et al. 2008), where dense plaques were shown to reach their maximum size in about a day and thereafter maintain a status quo.

The present study also goes forward to propose a reason for this discrepancy. Amyloid imaging through large open-skull cranial windows (as utilized solely by Meyer-Luehmann and colleagues) seems to activate gliosis, in contrast to thinned-skull windows of ≈1/10th the size, where calvaria are merely thinned down to allow in vivo microscopy without exposing the dura mater. This seems logical, as activation of gliosis has been shown in several studies to be an important factor in limiting plaque growth (Meyer-Luehmann et al. 2008; Bolmont et al., 2008; Yan et al., 2009). The stage of disease also seems to be important, as six-month-old mice with a higher proportion of smaller plaques demonstrate more accelerated plaque growth compared to 12-month-old animals (Yan et al., 2009).

Secondly, however carefully studies attempt to show that the sizes of the plaques estimated by in vivo imaging are true representatives of the plaques occurring at that or a later stage of disease, it is always difficult to do so. Lastly, it’s important to keep in mind that the methoxy-X04 used in multiphoton in vivo microscopy only binds to fibrillar Aβ and not to the soluble/oligomeric forms of Aβ that most likely provide the initial nidus of plaque formation. For this reason I don’t believe that we have had the final word on the kinetics and dynamics of plaque formation. Important from a therapy point of view is that anti-Aβ treatments have to be started as early as possible in order to be efficacious—that everyone agrees on.

View all comments by Samir Kumar-Singh

This is excellent work. The authors make elegantly the case that the procedures used to visualize amyloid plaques in vivo may strongly affect the generation and dynamics of the plaques. It is also of strong interest that interstitial Aβ peptide is such an important contributor to the plaque dynamics, as this is a rather small pool of total Aβ in the brain, and also highly dynamic and influenced by medication. Finally, the fact that 20-30 percent changes in that pool strongly affect the plaque formation should indeed raise hope that a therapeutic window exists for secretase inhibitors.

I strongly recommend the paper.

View all comments by Bart De Strooper

In my opinion, the discussion above misses one important fact: Brad Hyman's group published already in 2001 that plaques do not grow over time and that there is a restriction on plaque growth (Christie et al., 2001). In that study, more than 300 plaques were analyzed with two-photon microscopy over a time period of up to five months, and the investigators found the majority of plaques remained unchanged in size over time. Even more importantly, the data were observed using the thinned-skull method, i.e., the same method used by Yan et al., 2009. Therefore, thinned-skull versus open-skull preparation alone cannot account for the opposing result.

View all comments by Melanie Meyer-Luehmann

We appreciate the comments of Dr. Meyer-Luehmann. However, the absence of plaque growth reported in the Christie et al. (2001) paper is very consistent with the data reported in our recent paper (Yan et al, 2009). Although we observed marked plaque growth in six-month-old APP/PS1 mice (early in plaque pathogenesis), we saw little to no growth in 10-month-old APP/PS1 mice. Of note, the Christie et al. paper did not see plaque growth in 18-month-old (mean age) Tg2576 mice. Therefore, our observations in older animals who have more advanced pathology are in agreement with the Christie et al. paper.

References:
Christie RH, Bacskai BJ, Zipfel WR, Williams RM, Kajdasz ST, Webb WW, Hyman BT. Growth arrest of individual senile plaques in a model of Alzheimer's disease observed by in vivo multiphoton microscopy. J Neurosci. 2001 Feb 1;21(3):858-64. Abstract

View all comments by Jin-Moo Lee

This and other research demonstrates the deposition of Aβ in vivo in an animal model. Do we know that the “structures” that are shown being formed are also the same structures that are identified histo- or immunochemically postmortem?

Thus, are we confident that what is observed is the full process that results in the structures that we identify classically as plaques postmortem?

The alternative is that we are observing one part of a process. In some instances what is deposited may eventually be removed or transformed to something else and it is this “something else” which we identify postmortem as senile plaques.

Are senile (neuritic) plaques simply deposits of Aβ, or are they more than this?

View all comments by Chris Exley