Despite the significant increase in our understanding of mechanisms that lead to neuronal damage and memory loss in Alzheimer’s disease (AD), to date there are no effective ways to prevent or treat this devastating disease. Thus, identifying and validating novel therapeutic targets in AD remain major research goals. In this paper just published in Annals of Neurology, Himeno and coworkers have investigated the effects of apomorphine (APO) on neuropathological alterations and memory impairment in 3xTg mice. APO is a non-specific dopamine receptor agonist, exhibiting submicromolar affinities for both D1-type and D2-type dopamine receptors. In addition to its dopaminergic action, APO has been shown to protect neurons from oxidative stress in experimental models of Parkinson’s disease and stroke. Because oxidative stress is a prominent feature in AD brains, Himeno et al. tested the hypothesis that APO might protect 3xTg mice from amyloid-induced brain pathology and memory deficits. In line with their expectations, they found that Tg mice treated for one month with weekly injections...  Read more

Despite the significant increase in our understanding of mechanisms that lead to neuronal damage and memory loss in Alzheimer’s disease (AD), to date there are no effective ways to prevent or treat this devastating disease. Thus, identifying and validating novel therapeutic targets in AD remain major research goals. In this paper just published in Annals of Neurology, Himeno and coworkers have investigated the effects of apomorphine (APO) on neuropathological alterations and memory impairment in 3xTg mice. APO is a non-specific dopamine receptor agonist, exhibiting submicromolar affinities for both D1-type and D2-type dopamine receptors. In addition to its dopaminergic action, APO has been shown to protect neurons from oxidative stress in experimental models of Parkinson’s disease and stroke. Because oxidative stress is a prominent feature in AD brains, Himeno et al. tested the hypothesis that APO might protect 3xTg mice from amyloid-induced brain pathology and memory deficits. In line with their expectations, they found that Tg mice treated for one month with weekly injections of APO exhibited significantly improved performance in memory tasks compared to untreated Tg animals. Interestingly, they also found that treatment with APO stimulates the degradation of Aβ by insulin-degrading enzyme (IDE), thus reducing neuronal levels of amyloid peptide. As an added bonus, neuronal levels of phosphorylated tau (p-tau), a hallmark of AD pathology, were also reduced in APO-treated Tg mice.

Even though Himeno’s study suggests that neuroprotection by APO is largely due to its ability to block neuronal oxidative stress and reduce intraneuronal amyloid, there may be more to this story than first meets the eye. In a recently published study (Jurgensen et al., 2010), we showed that a specific D1-type dopamine receptor agonist blocked the removal of surface AMPA and NMDA receptors from synapses in cultured hippocampal neurons. Redistribution of AMPA and NMDA receptors from synapses is instigated by soluble Aβ oligomers, increasingly recognized as the proximal neurotoxins in AD, and is related to dephosphorylation of critical serine residues responsible for membrane insertion of the receptors. In the specific case of AMPA receptors, we showed that Aβ oligomers induce calcineurin-mediated dephosphorylation of Ser845 of the GluR1 subunit, causing the receptor to be removed from the membrane.

Because AMPA and NMDA receptors play key roles in synaptic plasticity, our findings provide a direct molecular mechanism to explain the inhibition of synaptic long-term potentiation (LTP) and the facilitation of long-term depression (LTD) induced by Aβ oligomers. Directly supporting and extending these findings, a paper just out in Nature Neuroscience (D’Amelio et al., 2010) showed a calcineurin-dependent decrease in phosphorylation of GluR1 at Ser845, followed by removal of AMPA receptors from synapses in Tg2576-APPSwe mice. Interestingly, both Jurgensen et al. and D’Amelio et al. showed that neither AMPA nor NMDA receptors are readily degraded upon their removal from the surface, suggesting that mechanisms capable of stimulating reinsertion of the receptors into the neuronal membrane could potentially counteract the negative impact of Aβ oligomers. To test this hypothesis, we asked whether a specific D1-receptor agonist could rescue neurons from Aβ oligomer-induced loss of surface receptors and inhibition of plasticity. The rationale for this was that D1 receptors are coupled to stimulatory G proteins that activate adenylate cyclase, leading to increased production of cAMP. In turn, cAMP activates protein kinase A (PKA), which phosphorylates AMPA and NMDA receptors at the serine residues that control membrane insertion.

Remarkably, we found that the D1 agonist effectively blocked dephosphorylation and removal of AMPA and NMDA receptors from the neuronal membrane, and prevented oligomer-induced inhibition of LTP in hippocampal slices. This suggests that specific activation of D1 dopamine receptors could be a novel therapeutic approach to prevent Aβ oligomer-induced synapse failure and memory loss in the early stages of AD.

Thus, in addition to the possible antioxidant action and the ability to increase Aβ degradation (as suggested by Himeno’s study), direct neurochemical effects of dopamine receptor agonists may be beneficial in AD. From a therapeutic point of view, however, APO would probably not be the best choice. As mentioned above, APO is a non-specific dopamine receptor agonist, acting on both D1 and D2 families of receptors (in fact, APO has higher affinities for D2-like than for D1-like receptors). Contrary to D1-like receptors, D2-like receptors are coupled to inhibitory G proteins, thus leading to reduced cAMP production and reduced PKA-dependent phosphorylation of AMPA or NMDA receptor subunits. Moreover, because D2 receptors are particularly enriched in the nigrostriatal pathway involved in motor control, use of a drug that activates D2 receptors in this brain region may lead to undesirable, or perhaps unacceptable, side effects. Yet another side effect of APO, related to activation of D2 receptors, is that it is a potent emetic. On the other hand, D1 receptors are enriched in projections from the ventral tegmental area to the hippocampus, a circuitry that is known to play important roles in learning and memory.

In conclusion, the possibility to combat synapse failure and memory loss by selective activation of D1 receptors should be further investigated as an approach to develop more effective treatments in AD.

References:
Jurgensen S, Antonio LL, Mussi GE, Brito-Moreira J, Bomfim TR, De Felice FG, Garrido-Sanabria ER, Cavalheiro EA, Ferreira ST. Activation of D1/D5 dopamine receptors protects neurons from synapse dysfunction induced by amyloid-{beta} oligomers. J Biol Chem. 2010 Nov 29. Abstract

D'Amelio M, Cavallucci V, Middei S, Marchetti C, Pacioni S, Ferri A, Diamantini A, De Zio D, Carrara P, Battistini L, Moreno S, Bacci A, Ammassari-Teule M, Marie H, Cecconi F. Caspase-3 triggers early synaptic dysfunction in a mouse model of Alzheimer's disease. Nat Neurosci. 2010 Dec 12. Abstract

View all comments by Sergio Ferreira

Very nice studies of how a classic small molecule with action at numerous receptors could be therapeutic for AD.

View all comments by Lon Schneider

Ten years ago, Gunnar Gouras began what has become a persistent call for attention to the potential importance of intraneuronal Aβ (Gouras et al., 2000). Since it is virtually impossible to be certain about which APP fragments are actually being observed in a histological setting, in an abundance of caution, herein we refer to this material as Aβ-like immunoreactivity (Aβ-LIR). In the intervening decade, Gouras has continued to find intraneuronal Aβ under every bed, and a number of other investigators (e.g., Oddo et al., 2003; LaFerla et al., 2007) have joined in with a chorus of endorsements of the “intraneuronal Aβ hypothesis.” Still, several fundamental questions have gone unaddressed, and these questions accrue new importance with the reports from several labs (including Gouras’s lab) that modulation of neurotransmission or autophagy stimulates clearance of intraneuronal Aβ, and that neurons disgorged of their Aβ-LIR material are happier and better functioning (Almeida et al., 2005; Almeida et al., 2006; Tampellini et al., 2007; Tampellini et al., 2009; Tampellini et al.,...  Read more

Ten years ago, Gunnar Gouras began what has become a persistent call for attention to the potential importance of intraneuronal Aβ (Gouras et al., 2000). Since it is virtually impossible to be certain about which APP fragments are actually being observed in a histological setting, in an abundance of caution, herein we refer to this material as Aβ-like immunoreactivity (Aβ-LIR). In the intervening decade, Gouras has continued to find intraneuronal Aβ under every bed, and a number of other investigators (e.g., Oddo et al., 2003; LaFerla et al., 2007) have joined in with a chorus of endorsements of the “intraneuronal Aβ hypothesis.” Still, several fundamental questions have gone unaddressed, and these questions accrue new importance with the reports from several labs (including Gouras’s lab) that modulation of neurotransmission or autophagy stimulates clearance of intraneuronal Aβ, and that neurons disgorged of their Aβ-LIR material are happier and better functioning (Almeida et al., 2005; Almeida et al., 2006; Tampellini et al., 2007; Tampellini et al., 2009; Tampellini et al., 2010; Himeno et al., 2010; Caccamo et al., 2010; Spilman et al., 2010).

Certainly, many labs, including our own (Petanceska et al., 2000; Gandy et al 2010), have observed striking intraneuronal pathology in transgenic mice. This is somewhat surprising since intraneuronal Aβ is difficult to recover from cultured cells, with the exception of systems employing Swedish APP (Haass et al., 1993) and perhaps with the important exception of cultured neurons (Turner et al., 1996). Compare this with the cultured media from these cells: There, newly generated Aβ is readily apparent (Haass et al., 1992). Why should we focus on the tiny amount of intraneuronal Aβ when clearly most Aβ is secreted? Further, when turning to human postmortem material, intraneuronal Aβ is observed occasionally at best. So, to recap, here are two strikes against intraneuronal Aβ: first, the tiny stoichiometry of retained versus secreted material, and second, the poor validation of intraneuronal Aβ-LIR from human AD neuropathology. To be fair, one could argue that the intraneuronal Aβ-LIR was first generated as secreted Aβ that has subsequently been endocytosed. Still, the poor validation in human material is worrisome.

Despite these concerns, there are reasons to find intraneuronal Aβ-LIR to be an attractive target. In our recently described “oligomer only” Dutch APP mice (Gandy et al., 2010), the only site of Aβ-LIR accumulation is intraneuronal. This being the case, we are left to ponder the possibility that the mouse model might be exaggerating an authentic step in human AD pathogenesis, but which, in humans, occurs rapidly and/or so early in the development of the disease, and/or at such low levels, that we have been unable to capture it in postmortem human brain. Provisionally, we consider that the Aβ oligomers we are measuring are residing amongst the intraneuronal Aβ-LIR. The idea that intraneuronal Aβ oligomerization is a key step in pathogenesis brings to mind one formulation of how plaques are formed and why cerebral amyloidosis is miliary, i.e., that tiny chunks of indigestible Aβ-LIR material are extruded from neurons and initiate plaque formation. Although this next, unfortunately apt comment will induce snickers, this process has occasionally been compared to defecation.

The link between diabetes and AD also appears to dovetail with this story. Insulin signaling is a dramatic stimulator of Aβ release (Gasparini et al., 2001; Liao and Xu, 2009), and metformin potentiates insulin-sensitive Aβ release (Chen et al., 2009) and may be associated with attenuated neuropathology (Beeri et al., 2008). It is difficult to avoid the formulation that insulin-stimulated secretion of Aβ appears to be a good thing. Tampellini et al (2009 and 2010) fits this model as well.

So, while under-represented in the conventional narrative of the APP/Aβ lifecycle and not obvious in human neuropathology, there appears to be good reason to consider the reduction of intraneuronal Aβ-LIR as a valid target for AD therapy, even if we can’t quite figure out exactly what subcellular events transpired to create the intraneuronal Aβ-LIR material in the first place.

References:
Almeida CG et al: Beta-amyloid accumulation in APP mutant neurons reduces PSD-95 and GluR1 in synapses. Neurobiol Dis 2005, 20(2):187-198. Abstract

Almeida CG, Takahashi RH, Gouras GK: Beta-amyloid accumulation impairs multivesicular body sorting by inhibiting the ubiquitin-proteasome system. J Neurosci 2006, 26(16):4277-4288. Abstract

Beeri MS, Schmeidler J, Silverman JM, Gandy S, Wysocki M, Hannigan CM, Purohit D, Lesser G, Grossman HT, Haroutunian V. Combination of insulin with other diabetes medication is associated with lower Alzheimer’s neuropathology. Neurology 2008; 71:750-757. Abstract

Caccamo A, et al. Molecular interplay between mammalian target of rapamycin (mTOR), amyloid-beta, and Tau: effects on cognitive impairments. J.Biol.Chem. 2010;285:13107-13120. Abstract

Chen Y, et al. Antidiabetic drug metformin (GlucophageR) increases biogenesis of Alzheimer's amyloid peptides via up-regulating BACE1 transcription. Proc.Natl.Acad.Sci.U.S.A. 2009;106:3907-3912. Abstract

Gandy S et al: Days to criterion as an indicator of toxicity associated with human Alzheimer amyloid-beta oligomers. Ann.Neurol. 2010, 68:220-230. Abstract

Gasparini L, et al. Stimulation of beta-amyloid precursor protein trafficking by insulin reduces intraneuronal beta-amyloid and requires mitogen-activated protein kinase signaling. J.Neurosci. 2001;21:2561-2570. Abstract

Gouras GK et al: Intraneuronal Abeta42 accumulation in human brain. Am J Pathol 2000, 156(1):15-20. Abstract

Haass C, et al. Amyloid beta-peptide is produced by cultured cells during normal metabolism. Nature 1992;359:322-325. Abstract

Haass C and Selkoe DJ. Cellular processing of beta-amyloid precursor protein and the genesis of amyloid beta-peptide. Cell 1993;75:1039-1042. Abstract

Himeno E, et al. Apomorphine treatment for Alzheimer's mice promoting amyloid-b degradation. Ann. Neurol. 2010; DOI: 10.1002/ana.22319. Abstract

LaFerla FM, Green KN, Oddo S: Intracellular amyloid-beta in Alzheimer's disease. Nat Rev Neurosci 2007, 8(7):499-509. Abstract

Liao FF and Xu H. Insulin signaling in sporadic Alzheimer's disease. Sci. Signal. 2009;2:pe36. Abstract

Oddo S et al: Triple-transgenic model of Alzheimer’s diseae with plaques and tangles: intracellular Abeta and synaptic dysfunction. Neuron 2003, 39:409-421. Abstract

Petanceska SS, et al. Mutant presenilin 1 increases the levels of Alzheimer amyloid beta-peptide Abeta42 in late compartments of the constitutive secretory pathway. J.Neurochem. 2000;74:1878-1884. Abstract

Spilman P, et al. Inhibition of mTOR by rapamycin abolishes cognitive deficits and reduces amyloid-beta levels in a mouse model of Alzheimer's disease. PLoS One 2010;5:e9979. Abstract

Tampellini D et al: Internalized antibodies to the Abeta domain of APP reduce neuronal Abeta and protect against synaptic alterations. J Biol Chem 2007, 282(26):18895-18906. Abstract

Tampellini D et al: Synaptic activity reduces intraneuronal Abeta, promotes APP transport to synapses, and protects against Abeta-related synaptic alterations. J Neurosci 2009, 29(31):9704-9713. Abstract

Tampellini D et al: Effects of synaptic modulation of beta-amyloid, synaptophysin, and memory performance in Alzheimer’s disease transgenic mice. J Neurosci 2010, 30(43):14299-14303. Abstract

Turner RS, et al. Amyloids beta40 and beta42 are generated intracellularly in cultured human neurons and their secretion increases with maturation. J.Biol.Chem. 1996;271:8966-8970. Abstract

View all comments by Samuel Gandy
View all comments by John Steele

Reply to comment by Sam Gandy and John Steele
I appreciate the interest in intraneuronal Aβ by Gandy and Steele, who have been contributing to this topic. I also welcome the comment, because interaction and questioning is so important. Yes, it has been a long road, and I will underscore that many other Alzheimer’s investigators have joined this pursuit; see our recent review (Gouras et al., 2010). I have also noticed an increasing “grassroots” interest in this topic, at the poster level at conferences and in ever more published papers, although it still has not quite reached many review articles at top-tier journals. The aim of this work is not to start a fringe topic that is irrelevant, but to move AD forward, and the resistance to change has been remarkable. To address the major issues raised by Gandy and Steele:

1. Lack of clear evidence in human AD brain: For me, intraneuronal Aβ actually all began with human brain. Looking through a microscope at sections from postmortem Down’s syndrome brains immuno-labeled with Aβ40 and 42 antibodies, I was struck by the...  Read more

Reply to comment by Sam Gandy and John Steele
I appreciate the interest in intraneuronal Aβ by Gandy and Steele, who have been contributing to this topic. I also welcome the comment, because interaction and questioning is so important. Yes, it has been a long road, and I will underscore that many other Alzheimer’s investigators have joined this pursuit; see our recent review (Gouras et al., 2010). I have also noticed an increasing “grassroots” interest in this topic, at the poster level at conferences and in ever more published papers, although it still has not quite reached many review articles at top-tier journals. The aim of this work is not to start a fringe topic that is irrelevant, but to move AD forward, and the resistance to change has been remarkable. To address the major issues raised by Gandy and Steele:

1. Lack of clear evidence in human AD brain: For me, intraneuronal Aβ actually all began with human brain. Looking through a microscope at sections from postmortem Down’s syndrome brains immuno-labeled with Aβ40 and 42 antibodies, I was struck by the marked intraneuronal labeling of particularly Aβ42 in Alzheimer’s prone neurons of already quite young Down’s syndrome brains; neurons of layer 2 in the entorhinal cortex and CA1 in the hippocampus were most prominently labeled. Thus, I would say that intraneuronal Aβ can be quite obvious in human brains. Many groups have now reported intraneuronal Aβ accumulation in human AD and Down’s syndrome brains. So why the concern about human brain by Gandy and Steele? I believe it is mainly because when you look at typical postmortem human AD brain (where plaque pathology tends to be advanced), intraneuronal Aβ42 is not as obvious. In fact, the same is seen with advanced plaque pathology in AD transgenic mice. We still do not have the full answer to this, but part of the answer—which we first realized with immuno-EM—is that most of the intraneuronal Aβ42 is present in neurites and synapses, which cannot be seen by light microscopy. In addition, conformational antibody specificities impact the amount of Aβ that one can see. For example, we have learned that standard Aβ42 antibodies are poor at detecting anything above dimers and trimers (they are best for monomers), limiting the amount of Aβ that can be detected with one antibody.

2. Lack of biochemical evidence: It is true that this was a major issue. In unpublished work, we struggled with this. For example, we could add a lot of synthetic Aβ1-42 to cell lysates, but subsequently could retrieve only a small fraction of this added Aβ. Given its hydrophobic nature, we may have to study Aβ42 more like a lipid. Since one cannot run a Western for lipids, the lipid field has increasingly turned to using fluorescence labeling—an area that has been revolutionized in recent years by better microscopes, cameras, and imaging software. When using immunofluorescence for intraneuronal Aβ, one needs to keep using the critical controls (see point 3, below). But to get back to biochemistry, we were excited by recent work from a biochemistry lab at the Karolinska Institute (Hashimoto et al., 2010); using laser capture microdissection microscopy in conjunction with a sensitive ELISA, they showed that AD-vulnerable CA1 neurons had a much higher ratio of Aβ42 to 40 in sporadic AD compared to control brains. In addition, while this higher ratio was maintained in cerebellar Purkinje neurons, the absolute levels of Aβ were much lower. In addition, Gylys, Cole, and colleagues have been showing nice evidence consistent with Aβ accumulation within synapses using synaptosomal preparations of AD brains (Gylys et al., 2004).

3. Secreted Aβ: I do not believe that it is as clear that secreted Aβ42 is so much more abundant than intraneuronal Aβ42. If we examine conditioned media of neurons grown at low density, we cannot even detect Aβ40, let alone Aβ42, while intraneuronal Aβ42 labeling is prominent. Again, the use of controls is critical: APP knockout neurons, in conjunction with biochemical antibody analysis, and also comparing Aβ labeling with antibodies directed at different domains of APP (including ones you believe the Aβ42 antibody may be labeling!).

Further, our immuno-EM actually argues against oligomerized Aβ42 being actively extruded from cells, as Gandy and Steele speculate. The subcellular morphology appears too badly damaged to allow for such active microtubule-based extrusion (Takahashi et al., 2004). The EM supports that this intraneuronal Aβ becomes extracellular following synaptic/neuritic degeneration.

Lastly, I will plug our recent paper that had received little attention, since I believe it is quite important. The main message is that we provide experimental evidence that plaque load does not correlate with Aβ-related destruction of synapses. The second message is that this paper counterbalances recent studies that have stressed the detrimental aspects of synaptic activity for AD. We show that reduced synaptic/cerebral activity destroys synapses in AD transgenic (but not wild-type mice), despite a decrease in plaques (whereas there is an increase in intraneuronal Aβ42). This might be important for clinical trials. Why? Because while amyloid imaging should be very helpful in diagnosing AD, we must be cautious using it as a surrogate marker for disease progression. We also hypothesize that intraneuronal Aβ is key to understanding why the aborted active vaccine trial showed continued cognitive decline despite plaque removal. Intraneuronal Aβ might also clarify reduced CSF Aβ42 in AD, which could be due to reduced release of the peptide from cells rather than accumulation in plaques. Our latest findings (Tampellini et al., 2010) support that synaptic activity has benefits in warding off AD. This doesn’t argue against the hypothesis that chronic sites of elevated synaptic/brain activity are prone to develop AD, or against detrimental effects of hyperexcitability, or sleep deprivation. We reconcile these various studies as pointing to Aβ altering the cellular machinery at synapses that no longer can effectively do what it could when young—efficiently secrete Aβ while also reducing intraneuronal Aβ42 with activity.

To conclude, after looking at enough electron micrographs of various AD transgenic mice, as well as advanced human AD brains, we see that intraneuronal accumulating and aggregating Aβ42 clearly appears to be a major problem, which is why we focus on it! The realization that this problem initiates within synapses (Takahashi et al., 2002) is what prompted us to turn to how accumulating Aβ alters synapses. At the same time, Aβ and synapses are, of course, not acting in isolation, and important contributions to AD pathogenesis come from ApoE, tau, inflammatory cells, the vasculature, etc., in addition to other factors involved in brain aging.

References:
Gouras GK, Tampellini D, Takahashi RH, Capetillo-Zarate E. Intraneuronal beta-amyloid accumulation and synapse pathology in Alzheimer's disease. Acta Neuropathol. 2010 May;119(5):523-41. Abstract

Hashimoto M, Bogdanovic N, Volkmann I, Aoki M, Winblad B, Tjernberg LO. Analysis of microdissected human neurons by a sensitive ELISA reveals a correlation between elevated intracellular concentrations of Abeta42 and Alzheimer's disease neuropathology. Acta Neuropathol. 2010 May;119(5):543-54. Abstract

Gylys KH, Fein JA, Yang F, Wiley DJ, Miller CA, Cole GM. Synaptic changes in Alzheimer's disease: increased amyloid-beta and gliosis in surviving terminals is accompanied by decreased PSD-95 fluorescence. Am J Pathol. 2004 Nov;165(5):1809-17. Abstract

Takahashi RH, Almeida CG, Kearney PF, Yu F, Lin MT, Milner TA, Gouras GK. Oligomerization of Alzheimer's beta-amyloid within processes and synapses of cultured neurons and brain. J Neurosci. 2004 Apr 7;24(14):3592-9. 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

Takahashi RH, Milner TA, Li F, Nam EE, Edgar MA, Yamaguchi H, Beal MF, Xu H, Greengard P, Gouras GK. Intraneuronal Alzheimer abeta42 accumulates in multivesicular bodies and is associated with synaptic pathology. Am J Pathol. 2002 Nov;161(5):1869-79. Abstract

View all comments by Gunnar K. Gouras

We commend Gunnar Gouras for persevering with his studies on intraneuronal amyloid-β (Aβ). Gouras and his collaborators pioneered the idea that the Alzheimer’s disease (AD)-related Aβ pathology begins inside the neuron, and precedes both neurofibrillary tangles and Aβ plaque deposition (1). Over more than a decade, Gouras, and a few others, continued to gather evidence that the initial observations made in the AD brain hold true in mouse models of AD and cultured neurons, where they can be studied. The problem of intracellular Aβ will certainly preoccupy more and more investigators in the years to come. It also preoccupies us (2). Although views may vary, this Aβ accumulates within neurons early during AD and appears to correlate well with the incipient phases of the disease. While not definitively proven, it is likely that this intraneuronal Aβ contributes to the synapse pathology in AD, as Gouras correctly argues (3); it may also constitute the seed that nucleates AD plaques (see, e.g., 4).

In the CAD neuronal cell line—one of our systems of study—intracellular Aβ...  Read more

We commend Gunnar Gouras for persevering with his studies on intraneuronal amyloid-β (Aβ). Gouras and his collaborators pioneered the idea that the Alzheimer’s disease (AD)-related Aβ pathology begins inside the neuron, and precedes both neurofibrillary tangles and Aβ plaque deposition (1). Over more than a decade, Gouras, and a few others, continued to gather evidence that the initial observations made in the AD brain hold true in mouse models of AD and cultured neurons, where they can be studied. The problem of intracellular Aβ will certainly preoccupy more and more investigators in the years to come. It also preoccupies us (2). Although views may vary, this Aβ accumulates within neurons early during AD and appears to correlate well with the incipient phases of the disease. While not definitively proven, it is likely that this intraneuronal Aβ contributes to the synapse pathology in AD, as Gouras correctly argues (3); it may also constitute the seed that nucleates AD plaques (see, e.g., 4).

In the CAD neuronal cell line—one of our systems of study—intracellular Aβ (monomeric and oligomeric) is readily detectable in immunoblots (2), while secreted Aβ is more difficult to detect—in spite of the abundance of secreted sAPPs. This situation is similar to what Gouras noticed in his work with neuronal cultures (see his comment to this paper). This result suggests that Aβ accumulates and oligomerizes within neurons prior to its accumulation in the extracellular space. Intuitively, this should be so. Although Aβ can be cleaved off the recycled CTFs at the cell surface, and thus released without being truly secreted, a large body of evidence points to the fact that most Aβ is generated within the cell. Under normal conditions, most of it is probably degraded within the cell, and some is retrieved from the compartments where it was produced and released into the extracellular space. In AD, a larger fraction may be retained within the cell. Most importantly—as studies by Randy Nixon and collaborators consistently show—a larger fraction may not be properly degraded due to a failure of the endosome/lysosome/autolysosome system (5-10). In the end, the intracellularly accumulated Aβ becomes extracellular by a variety of mechanisms, including decomposition of the dying neuron.

Animal models of disease, cultured neurons, and in vitro reconstitutions are the only systems available to researchers for dissecting the mechanisms of AD by experimentation. Obviously, if the data are not consistent with what is known about the human disease, they are discarded. This is certainly not the case with intracellular Aβ, which is present in the AD brain (1) in the soma and particularly in dystrophic neurites, mostly in endosomal/lysosomal and autophagic compartments (7). The study of intraneuronal Aβ and, in particular, of the mechanism of oligomerization and accumulation of Aβ within neurons—with animal models and cultured cells—will certainly contribute to the elucidation of the pathogenic mechanisms in AD.

References:
1. Gouras, G.K., et al., Intraneuronal Abeta42 accumulation in human brain. Am J Pathol, 2000 156(1): p. 15-20. Abstract

2. Muresan, Z. and V. Muresan, Neuritic Deposits of Amyloid-{beta} Peptide in a Subpopulation of Central Nervous System-Derived Neuronal Cells. Mol Cell Biol, 2006 26(13): p. 4982-97. Abstract

3. Gouras, G.K., et al., Intraneuronal beta-amyloid accumulation and synapse pathology in Alzheimer's disease. Acta Neuropathol, 2010. 119(5): p. 523-41. Abstract

4. Muresan, Z. and V. Muresan, Seeding Neuritic Plaques from the Distance: A Possible Role for Brainstem Neurons in the Development of Alzheimer's Disease Pathology. Neurodegenerative Dis, 2008 5(3-4): p. 250-3. Abstract

5. Yu, W.H., et al., Macroautophagy—a novel Beta-amyloid peptide-generating pathway activated in Alzheimer's disease. J Cell Biol, 2005 171(1): p. 87-98. Abstract

6. Nixon, R.A., et al., Extensive involvement of autophagy in Alzheimer disease: an immuno-electron microscopy study. J Neuropathol Exp Neurol, 2005 64(2): p. 113-22. Abstract

7. Cataldo, A.M., et al., Abeta localization in abnormal endosomes: association with earliest Abeta elevations in AD and Down syndrome. Neurobiol Aging, 2004 25(10): p. 1263-72. Abstract

8. Ginsberg, S.D., et al., Regional selectivity of rab5 and rab7 protein upregulation in mild cognitive impairment and Alzheimer's disease. J Alzheimers Dis, 2010. 22(2): p. 631-9. Abstract

9. Ginsberg, S.D., et al., Microarray analysis of hippocampal CA1 neurons implicates early endosomal dysfunction during Alzheimer's disease progression. Biol Psychiatry, 2010. 68(10): p. 885-93. Abstract

10. Lee, J.H., et al., Lysosomal proteolysis and autophagy require presenilin 1 and are disrupted by Alzheimer-related PS1 mutations. Cell, 2010. 141(7): p. 1146-58. Abstract

View all comments by Zoia Muresan
View all comments by Virgil Muresan