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Originally published In Press as doi:10.1074/jbc.M507892200 on November 10, 2005

J. Biol. Chem., Vol. 281, Issue 3, 1599-1604, January 20, 2006
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Temporal Profile of Amyloid-beta (Abeta) Oligomerization in an in Vivo Model of Alzheimer Disease

A LINK BETWEEN Abeta AND TAU PATHOLOGY*

Salvatore Oddo{ddagger}, Antonella Caccamo{ddagger}, Levina Tran{ddagger}, Mary P. Lambert§, Charles G. Glabe¶, William L. Klein§, and Frank M. LaFerla{ddagger}1

From the {ddagger}Department of Neurobiology and Behavior, Department of Molecular Biology and Biochemistry, University of California, Irvine, California 92697-4545 and the §Department of Neurobiology and Physiology, Northwestern University, Evanston, Illinois 60208

Received for publication, July 20, 2005 , and in revised form, October 3, 2005.


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
Accumulation of amyloid-beta (Abeta) is one of the earliest molecular events in Alzheimer disease (AD), whereas tau pathology is thought to be a later downstream event. It is now well established that Abeta exists as monomers, oligomers, and fibrils. To study the temporal profile of Abeta oligomer formation in vivo and to determine their interaction with tau pathology, we used the 3xTg-AD mice, which develop a progressive accumulation of plaques and tangles and cognitive impairments. We show that SDS-resistant Abeta oligomers accumulate in an age-dependent fashion, and we present evidence to show that oligomerization of Abeta appears to first occur intraneuronally. Finally, we show that a single intrahippocampal injection of a specific oligomeric antibody is sufficient to clear Abeta pathology, and more importantly, tau pathology. Therefore, Abeta oligomers may play a role in the induction of tau pathology, making the interference of Abeta oligomerization a valid therapeutic target.


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
Alzheimer disease (AD)2 is the most common neurodegenerative disorder, affecting ~5 million Americans (1). Neuropathologically, it is characterized by the accumulation of extracellular plaques, mainly comprised of a small peptide called amyloid-beta (Abeta), and intracellular neurofibrillary tangles, consisting of aggregates of hyperphosphorylated tau protein (2). Based on compelling genetic evidence, it has been postulated that pathological assemblies of Abeta are the cause of all forms of AD (3), whereas tau pathology and other neuropathological changes are a downstream consequence of the pathological accumulation of Abeta species. This hypothesis has received strong experimental support from studies of various transgenic models of AD (4–6). The source of Abeta that initiates the neurodegenerative process, however, remains unknown. Traditionally, Abeta has been viewed as being generated and secreted extracellularly, but it is also becoming increasingly apparent that some Abeta can be generated in different intracellular compartments, such as the endoplasmic reticulum and the trans-Golgi (7–11). Moreover, there is mounting evidence to support a pathophysiologic role for intracellular Abeta in AD an Down syndrome (see Ref. 12 for review).

Abeta exists in several different physical states, including as monomers, oligomers, or fibrils. Evidence from in vitro studies demonstrates that synthetic Abeta monomers aggregate in a time-dependent fashion to form oligomers, which eventually may form fibrils (13–15). During the last decade, in vitro and in vivo experimental evidence points to soluble Abeta oligomers, also referred to as Abeta-derived diffusible ligands, as the predominant neurotoxic species for neurons (16, 17). In this regard, Abeta oligomers are very potent toxic species, as even nanomolar concentrations have been shown to kill mature neurons in hippocampal slices (18). Moreover, Abeta oligomers appear to interfere with many critical neuronal activities, including inhibiting long term potentiation (LTP) in organotypic hippocampal slices (18, 19). Abeta oligomers can also cause calcium dysregulation and membrane disruption, thus interfering with overall cell functioning (20, 21). The toxicity of Abeta oligomers has also been shown in vivo. In particular, intracerebroventricular injection of oligomers inhibits LTP (22) and specifically disrupts cognitive function (23). Importantly, the concomitant injection of the anti-Abeta antibody 6E10 with Abeta oligomers neutralizes the oligomer-induced LTP dysfunction (24). These data strongly support the idea that oligomers represent a fundamental species responsible for mediating Abeta toxicity in AD (2, 16).

Given the critical role that Abeta oligomers appear to play in the pathogenesis of AD, we sought to understand their genesis in an in vivo model of AD. We used the 3xTg-AD mice, which develop an age-dependent accumulation of both plaques and tangles in AD-relevant brain regions (25). We first charted the temporal relationship between Abeta monomers, oligomers, and fibrils formation. Of particular relevance, we found that oligomerization of Abeta appears to commence intraneuronally in the 3xTg-AD brains, a finding consistent with other in vitro and in vivo studies (26–28). As we previously showed that the administration of Abeta-specific antibodies reduces not only the Abeta pathology but also early forms of tau pathology (6), here we extended these studies and showed that the administration of an Abeta oligomeric-specific antibody is also able to reduce the tau pathology in these mice. These findings suggest that the action of Abeta oligomers may be a key event in the initiation of the tau pathology.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 REFERENCES
 
Mice—We previously described the generation of the 3xTg-AD mice (25). Briefly, the 3xTg-AD mice were derived by co-microinjecting two independent transgenes encoding human APPSwe and the human tauP301L (both under control of the mouse Thy1.2-regulatory element) into single-cell embryos harvested from homozygous mutant PS1M146V knock-in (PS1-KI) mice.

Immunoblotting—Brains from 3xTg-AD and NonTg mice were homogenized in 2% SDS supplemented with a complete miniprotease inhibitor tablet (Roche Diagnostics). The homogenized mixes were briefly sonicated to sheer the DNA and centrifuged at 4 °C for 1 h at 100,000 x g. The supernatant was used for dot blot and Western blot. Protein concentration was determined using the Bio-Rad protein assay, and samples were adjusted with 2% SDS to the same concentration. For the dot blot, brain extracts were applied to a nitrocellulose membrane and air-dried. For the Western blot, brain extracts were heated in loading buffer (NuPAGE LDS sample buffer from Invitrogen) for 10 min at 70 °C and resolved by SDS-PAGE (10% Bis-Tris from Invitrogen) under reducing conditions and transferred to a nitrocellulose membrane. Membranes were incubated in a 5% solution of nonfat dry milk for 1 h at 20 °C. After overnight incubation at 4 °C with primary antibody, the blots were washed in Tween 20-TBS (T-TBS) (0.02% Tween 20, 100 mM Tris, pH 7.5, 150 nM NaCl) for 20 min and incubated at 20 °C with the appropriate secondary antibody for 1 h. The blots were washed in T-TBS for 20 min and incubated for 5 min with Super Signal (Pierce), washed, and exposed.


Figure 1
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  		FIGURE 1.
SDS-resistant oligomers accumulate in an age-dependent fashion in the 3xTg-AD mice. A, proteins extracted from brains of the 3xTg-AD and NonTg mice of various ages were homogenized in 2% SDS and applied to a nitrocellulose membrane, which was subsequently probed either with oligomer-specific antibodies M71/3 or A11. B, quantification of the dot blots shown in A (p < 0.0001 and < 0.005 for M71/3 and A11, respectively, using one-way analysis of variance). For the quantification of the dot blot, the raw values obtained for M71/3 and A11 samples were adjusted with the values obtained with the samples in the NonTg mice. At 2 months of age, there is only faint staining in the 3xTg-AD samples, which is similar to that found in NonTg mice at all the ages. At 6 months of age, there is a marked and significant increase in Abeta oligomer levels for both M71/3 and A11 (p < 0.05, t test analysis); however, these levels are significantly lower at 9 months for both M71/3 and A11 (p < 0.05, t test analysis). The reason underlying this decrease is not clear; however, from 12 to 20 months of age there is an age-dependent accumulation of Abeta oligomers, with a significant increase between 12 and 15 months for both M71/3 and A11 (p < 0.05, t test analysis). The error bars represent S.E. C, Western blot of proteins extracted in 2% SDS from 3xTg-AD and NonTg mice and probed with antibody 6E10. SDS-resistant oligomers were detected in 20-month-old mice and are noted by the molecular weights reported between the two blots. The arrows point to a nonspecific band detected by 6E10 in both 3xTg-AD and NonTg mice. The star indicates C99, which accumulates in an age-dependent fashion in the 3xTg-AD mice.

 
Immunohistochemistry—Brains from 3xTg-AD and NonTg mice were fixed for 48 h in 4% paraformaldehyde. Free-floating sections (50-µm thick) were obtained using a vibratome slicing system (Pelco, Redding, CA) and stored in 0.02% sodium azide in phosphate-buffered saline. The endogenous peroxidase activity was quenched for 30 min in 3% H2O2. Sections were then incubated in 90% formic acid for 7 min to expose the epitope. The appropriate primary antibody was applied overnight at 4 °C. Sections were washed with TBS and incubated with the appropriate secondary antibody for 1 h at 20°C. Sections were developed with diaminobenzidine substrate using the avidin-biotin horseradish peroxidase system (Vector Laboratories, Burlingame, CA). For confocal microscopy, after incubating the sections in the appropriate primary antibody, suitable Alexa Fluor secondary antibody (Molecular Probes) was applied. Subsequently, sections were washed in phosphate-buffered saline, mounted on slide and coverslip with Vectashield (Vector Laboratories).

Surgeries—3xTg-AD and NonTg mice (12-months old) were anesthetized with 0.6 ml/25 g of body weight of avertin (1.3% tribromoethanol, 0.8% amylalcohol) and placed in a stereotactic apparatus (MyNeuroLab, St. Louis, MO) with a mouse adaptor. 2 µg of the A11 antibody, an isotype control antibody (anti-protozoan) or phosphate-buffered saline were injected into the left hippocampus through a 33-gauge injector attached to a 5-µl Hamilton syringe (Hamilton Company, Reno, NV). The coordinates, with respect to bregma, were -2.7 mm posterior, +2.5 mm lateral, and -2.0 mm ventral to the skull. The rate of injection was 1 µl/min, after which the cannula was left in place for an additional 5 min to allow for diffusion. Animals were kept on a warming pad until they had fully recovered from anesthesia and were kept in individual cages until they were sacrificed for tissue processing to prevent damage to the scalp sutures. All animal procedures were in accordance with the National Institute of Health Guide for the Care and Use of Laboratory Animals and approved by the UCI IACUC. All appropriate measures were taken to minimize pain and discomfort in experimental animals.

Statistical Analysis—Data were subsequently analyzed by analysis of variance or t test comparison, using Graphpad Prism software.


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 REFERENCES
 
The development of oligomeric-specific antibodies has greatly facilitated the understanding of the role of oligomers in the pathogenesis AD. In this study we used antibody M71/3, which is highly sensitive and detects minute amounts of low molecular weight Abeta oligomers (12–24 mers) (29, 30), and antibody A11, which is selective for higher molecular weight Abeta oligomers (31). Neither of these antibodies cross-react with Abeta monomers or natively folded amyloid precursor protein (29, 31).

Abeta Oligomers Are First Detected Intraneuronally—We first defined the temporal profile of Abeta oligomers in the brain of the 3xTg-AD mice using both biochemical and immunohistochemical methods. Brains from 3xTg-AD and age-matched NonTg were extracted in 2% SDS, and the resultant proteins were applied to a nitrocellulose membrane, which was subsequently probed with the oligomer-specific antibodies M71/3 and A11. At 2 months of age, the signal intensity was comparable between the 3xTg-AD and NonTg samples, suggesting that Abeta oligomers have not yet built up to any significant extent (Fig. 1A). Although the NonTg signal remained low across all time points, there was an age-dependent increase in M71/3- and A11-specific immunoreactivity in the 3xTg-AD samples, first apparent by 6 months of age (Fig. 1, A and B). Thus, oligomerization of Abeta starts between 2 and 6 months of age. Interestingly, we found that the intensity of both M71/3 and A11 immunoreactivity was significantly lower at 9 and 12 months of age compared with the 6 months time point (Fig. 1, A and B). Although the reason for this decrease in oligomer levels during these ages is not clear, it may be because of an increase in Abeta fibrillarization that occurs at these ages (see below). This transient reduction occurs within a narrow time frame as between 12 and 20 months, we again found an age-dependent increase in Abeta oligomers in the 3xTg-AD brains (Fig. 1, A and B).


Figure 2
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  		FIGURE 2.
Abeta oligomers are first detected intraneuronally. Sections from the hippocampi of 3xTg-AD mice were stained with antibody 6E10 (which recognizes all forms of Abeta), antibody M71/3 (which recognizes Abeta oligomers), and thioflavine S that is specific for fibrils. A–C, in 4-month-old mice, intraneuronal 6E10 staining is readily apparent, but there is no staining with either M71/3 or thioflavine, indicating that at this age Abeta is predominantly monomeric. D–F, at 6 months of age, there is an increase in 6E10 staining, which correlates with the appearance of intraneuronal staining with the M71/3 antibody. The lack of thioflavine S reactivity indicates that the material recognized by M71/3 is Abeta oligomers. G–I,at12 months of age, both intra- and extracellular 6E10 staining is apparent. At this age, extracellular M71/3 and thioflavine-positive structures are apparent. Note that the M71/3 changes from predominantly intracellular at 6 months to predominantly extracellular at 12 months. J–L, at 20 months of age, there is a robust staining with all three markers, indicative of an advanced Abeta pathology.

 
Abeta oligomers appear to be relatively stable structures, as SDS-resistant species have been isolated from post-mortem human brains and cell lines (22, 29, 32, 33). We determined whether SDS-resistant Abeta oligomers could also be isolated from the 3xTg-AD brains and resolved by SDS-PAGE and Western blotting. Following preparation of protein extracts in 2% SDS, we detected SDS-resistant Abeta dimers, trimers, and pentamers in brain samples from 20-month-old mice by Western blot using 6E10 (Fig. 1C). We were unable to detect Abeta (monomeric or oligomeric) in younger mice using this particular immunoblotting approach, perhaps because Abeta oligomers are not yet SDS-resistant and/or their presence is below threshold for detection by a Western blot. These data indicate that the SDS-resistant oligomers build-up in an age-dependent fashion in the 3xTg-AD mice and are readily detectable in older mice.

Results from several in vitro studies suggest that Abeta oligomers may be a transient intermediate stage leading to plaque formation; however, recent studies have also raised the possibility that Abeta oligomers could be more stable and not convert into fibrils (34). To determine the temporal relationship between these two Abeta species in vivo, we charted their accumulation in the hippocampus of the 3xTg-AD mice using antibody 6E10 (which detects all forms of Abeta), M71/3 (which detects low molecular weight Abeta oligomers at the fentomole levels) (30), and thioflavine S staining (to identify fibrillar Abeta deposits). We focused on the hippocampus because this brain region is highly affected by Abeta pathology in both AD and in the 3xTg-AD mice; moreover, the well defined structural organization of the hippocampus facilitates the study of the temporal progression of Abeta aggregation. At 4 months of age, robust intraneuronal staining was apparent with antibody 6E10, but not with M71/3 or thioflavine S (Fig. 2, A–C), suggesting that the overwhelming majority of Abeta exists in a soluble, monomeric state at this age. At 6 months of age, 6E10 staining increased (Fig. 2D), and intraneuronal M71/3 immunoreactivity first becomes apparent (Fig. 2E), suggesting that oligomerization of Abeta occurs intraneuronally between 4–6 months of age (see also Fig. 3A). Notably, by 12 months, intraneuronal Abeta oligomers are no longer apparent in the CA1/subiculum, although we found a marked increase in M71/3 extracellular staining (c.f. Fig. 2, I and H). Because the M71/3 antibody does not discriminate between oligomers and fibrils, we cannot exclude the possibility that some of the positive material shown in Fig. 2H is fibrillar in nature. However, staining of an adjacent section with thioflavine S (Fig. 2I) reveals far less staining compared with that detected by M71/3, suggesting that the majority of the extracellular deposits shown in Fig. 2H is most likely extracellular Abeta oligomers. Thus, there is an age-dependent shift in M71/3 immunoreactivity in the CA1/subiculum region from predominantly intracellular to predominantly extracellular staining between 6 and 12 months. Please note that protein samples shown in the dot blot in Fig. 1 were prepared in 2% SDS, which may have disrupted some Abeta structures, thereby accounting for the lower levels at this time point compared with the immunohistochemistry data. The presence of extracellular Abeta oligomers appears to occur prior to extracellular thioflavine-positive plaques (Fig. 2I), providing correlative in vivo evidence that oligomers represent an intermediate step leading to plaque formation. However, these data do not exclude the possibility that a fraction of Abeta oligomers can be more stable structures or that these structures can be independent of fibrillarization.


Figure 3
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  		FIGURE 3.
Intraneuronal oligomers show different conformations. Sections from hippocampus (A–C) or cortex (D–F) of the 3xTg-AD mice were double stained with M71/3 and a C-terminal Abeta42-specific antibody and analyzed using confocal microscopy. As expected, a proportion of the deposits positive with the Abeta42-specific antibody are also positive for M71/3, as this antibody recognizes monomeric Abeta as well. Surprisingly, however, we found a proportion of M71/3-positive structures that are negative for Abeta42-specific antibodies (green staining in C), suggesting that intraneuronal oligomers exist in different conformations, one where the C-terminal of Abeta is accessible, and one where it is not. Original magnification for A–C is 80x and for D–F is 20x.

 
To better analyze the conformation of intraneuronal Abeta oligomers, we double labeled sections from the 3xTg-AD mice with M71/3 and Abeta42-specific antibodies and analyzed them using confocal microscopy. We found that the oligomeric Abeta staining in the soma was very punctuate, indicating that these structures are located in an intracellular compartment (Fig. 3A). A similar staining pattern was detected with the Abeta42-specific antibody (Fig. 3B). We found that only a proportion of deposits detected by the Abeta42-specific antibody were also immunoreactive for M71/3, as Abeta-specific antibodies recognize monomeric Abeta as well (Fig. 3C). Surprisingly, however, we found a proportion of M71/3-positive structures that were negative for Abeta specific antibodies (Fig. 3C). These data suggest that intraneuronal oligomers exist in different conformations, one where the C-terminal of Abeta is accessible, and one where it is not. It is possible that these different conformations represent different stages of Abeta aggregation, and further biochemical experiments will be necessary to better address these conformational changes.

Co-localization of Abeta Oligomers with Early, Somatodendritic Tau but Not Late Hyperphosphorylated Tau—Recent evidence suggests that Abeta and tau pathology may influence each other. As part of our efforts to discern the nature of this interaction and to establish which form of Abeta may be involved, we first determined whether there was co-localization between Abeta oligomers and tau in vivo. Sections from 6- and 20-month-old 3xTg-AD mice were double-labeled with M71/3 and anti-tau antibodies. These ages were selected because tau accumulation in the somatodendritic compartment is the earliest sign of tau pathology in the 3xTg-AD mice, which is first apparent by 6 months of age. By 20 months, the tangle pathology is quite advanced and apparent with different silver stains and also immunoreactive with several phosphospecific tau antibodies (e.g. AT8, PHF-1) (25). Our analysis indicates that there is some but not complete co-localization between the M71/3 and HT7 immunoreactivity (Fig. 4, A–C). In contrast we found scant evidence of M71/3 and AT8 co-localization in 20-month-old mice (Fig. 4, D–F). These results indicate that Abeta oligomers co-localize with HT7-positive somatodendritic tau deposits. The co-localization of Abeta oligomers with early tau pathology may be fortuitous or given the growing appreciation that Abeta can induce the tau pathology, it is tempting to speculate that Abeta oligomers acting either intraneuronally or extraneuronally at the synaptic surface may contribute to the development of the tau pathology.


Figure 4
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  		FIGURE 4.
Abeta oligomers co-localize with early somatodendritic but not late hyperphosphorylated tau. Sections from 6-month-old 3xTg-AD mice (A–C) were stained with M71/3 and HT7 and analyzed using confocal microscopy. The somatodendritic localization of tau that is apparent following staining with antibody HT7 is generally regarded as an early stage of tau pathology. We found a good overlapping staining between M71/3 and HT7. Sections from 20-month-old 3xTg-AD mice (D–F) were double labeled with M71/3 and AT8. At this age the 3xTg-AD mice show advance tangle pathology. No co-localization was observed between these two antibodies. Original magnification for A–F is 40x.

 
Anti-oligomeric Abeta Antibody Clears Tau Pathology as Well as Abeta Pathology—We previously showed that a single injection of an anti-Abeta antibody clears Abeta deposits from the brains of the 3xTg-AD mice, which consequently leads to the reduction in early tau pathology (6). As it is plausible that only certain Abeta species may also result in clearance of tau, we determined the effects of an Abeta oligomeric-specific antibody on the clearance of the Abeta and tau pathology. We injected 2 µg of antibody A11 into the hippocampus of 12-month-old hemizygous 3xTg-AD mice. This antibody was used because it is highly selective for oligomeric Abeta, whereas M71/3 does not discriminate between oligomers and fibrils. We found that 7 days after a single administration of antibody A11, the Abeta pathology was reduced in the areas surroundings the injection site (Fig. 5, A–C). Remarkably, the removal of Abeta oligomers led to the clearance of the early tau pathology in the somatodendritic compartment of CA1 neurons (c.f. Fig. 5D–F). No clearance of Abeta or tau pathology was observed in the uninjected, contralateral hippocampus that served as an internal control (Fig. 5, A and D). We also injected 3xTg-AD mice with phosphate-buffered saline or an isotype control antibody and found that neither of these treatments had any effects on the Abeta and tau pathology (data not shown). These data strongly suggest that Abeta oligomers may be link between Abeta and tau pathology. Thus, besides impairing LTP (18, 22) and cognitive functions (23), Abeta oligomers can interact with tau pathology.


Figure 5
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  		FIGURE 5.
Anti-oligomeric antibody clears tau pathology as well as Abeta pathology. 2 µg of the Abeta-oligomeric antibody A11 were injected into the left hippocampus of 12-month-old 3xTg-AD hemizygous mice (n = 4/group). 7 days later their brains were analyzed by immunohistochemistry. A and B, horizontal sections were stained with different anti-Abeta antibodies, and we found a marked decrease in Abeta deposits in the ipsilateral side, whereas the contralateral, uninjected hippocampus was unaltered. C, quantification analysis shows the profound reduction in Abeta immunoreactivity in the ipsilateral compared with the contralateral hippocampus (p < 0.001). D and E, serial sections to those presented in A and B were stained with HT7. F, the ipsilateral side presented a greatly reduction in tau immunoreactivity compared with the contralateral side (p < 0.001).

 

   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
REFERENCES
 
One of the earliest clinical manifestations that occurs in AD is a profound memory loss. Recently, it has been proposed that this initial memory loss may be caused by synaptic failure, which may be a consequence of the accumulation of soluble Abeta oligomers (35, 36). This hypothesis is further strengthened by the isolation of oligomers from AD brains (29, 32). In particular, McLean and colleagues (32) showed that this soluble form of Abeta greatly correlates with the severity of the disease. Abeta oligomers isolated from AD brains, human cerebrospinal fluid, or cell lines are resistant to common solvents such as SDS (28, 29, 32, 33, 37). In this study, we showed that the 3xTg-AD mice develop SDS-resistant oligomers in a progressive manner, suggesting that these structures are fairly stable. This is notable considering that recently it has been proposed that Abeta oligomers may not just be a transient stage to fibrils, but they maybe metastable structures that interfere with neuronal function (34).

One of the major findings of this work was that Abeta oligomerization starts intraneuronally. These data are consistent with previous data showing that Abeta oligomerization also begins intracellularly in primary human neurons and in other neuronal cell lines and in transgenic mice (27, 28). Recent studies have shown that intraneuronal Abeta plays a pathophysiological role in the progression of the disease (reviewed in Ref. 12). In particular, we previously showed that the 3xTg-AD mice have a selective loss of {alpha}7 nicotinic acetylcholine receptors ({alpha}7nAChRs) restricted to brain regions that accumulate intraneuronal Abeta (38). Moreover, we showed that 6-month-old 3xTg-AD mice have profound LTP and cognitive deficits caused by intraneuronal Abeta (25, 39). Taken together, these data strongly argue that intraneuronal Abeta oligomers may be responsible for the loss of {alpha}7nAChRs and the LTP and cognitive deficits in the 3xTg-AD mice. On the other hand, there is also evidence that Abeta oligomers can act as extracellular ligands for synapses (29, 40). These two views are not mutually exclusive. Considering the potency of Abeta oligomers, it is possible that these structures are bioactive within the cell, interfering with cell function, and also, once in the extracellular space (Fig. 2), they can act as a ligand and interfere with general neuronal networking. Moreover, we cannot exclude the possibility that some Abeta oligomers may also form extracellularly and further contribute to Abeta toxicity and fibril formation. Additional studies will be necessary to determine the extent of the contribution of Abeta oligomers to the loss of {alpha}7nAChRs and the LTP and cognitive impairments in the 3xTg-AD mice. Nevertheless, the data presented here, along with other studies showing that Abeta oligomers impair LTP and cognitive functions (18, 19, 22, 23), strongly argue in favor of this hypothesis.

We and others presented data to show that Abeta and tau pathologies interact in vivo (4–6). In particular, we have shown that anti-Abeta antibodies clear both intracellular and extracellular Abeta deposits and lead to the subsequent clearance of the early tau pathology (6). In this study, we showed that a single intrahippocampal injection of the oligomeric-specific antibody A11 reduces intracellular Abeta deposits. In this regard, we have previously shown that intraneuronal Abeta deposits can also be removed by other anti-Abeta antibodies (6, 39). The mechanism underlying the removal of intraneuronal Abeta seems to be because of a dynamic relationship between intraneuronal and extracellular pools of Abeta (41). Notably, the injection of the anti-oligomeric Abeta antibody A11, leads to the reduction of tau pathology as well as Abeta pathology. These data strongly suggest that Abeta oligomers may represent a link between Abeta and tau pathology.

It has recently been shown that Abeta immunotherapy neutralizes Abeta oligomers, thus rescuing the oligomer-induced LTP deficit (24). Furthermore, Walsh and colleagues (42) showed that the oligomer-induced LTP deficit can be rescued by small molecules that block Abeta oligomerization. In consideration of these data and the findings presented here, it is plausible to speculate that blocking Abeta oligomerization is a valid therapeutic target to lessen or halt AD neurodegeneration, as preventing Abeta oligomer formation can rescue the LTP and behavioral impairments and the early tau pathology. This last point is very important as tau pathology is sufficient to cause neurodegeneration in humans; therefore, for any AD therapeutic intervention to be suitable, it needs to remove/halt the progression of tau pathology as well as Abeta pathology.


   FOOTNOTES
 
* This work was supported by Grant AG0212982 from the NIA, National Institutes of Health (to F. M. L.), Grants AG 18877, AG 22547, and AG 11385 from the National Institutes of Health (to W. L. K.), and Grant AG00538 from the Larry L. Hillblom Foundation and National Institutes of Health (to C. G. G.). The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact. Back

1 To whom correspondence should be addressed: Dept. of Neurobiology and Behavior, University of California, Irvine, 1109 Gillespie Neuroscience Bldg. Irvine, CA 92697-4545. Tel.: 949-824-1232; Fax: 949-824-7356; E-mail: laferla{at}uci.edu.

2 The abbreviations used are: AD, Alzheimer disease; Abeta, amyloid-beta; LTP, long term potentiation. Back



   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 

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