Alzheimer’s disease (AD) is an incurable terminal neurodegenerative disorder primarily affecting the elderly. Even after a century of intensive investigation, its pathogenic mechanism still remains enigmatic. Many hypotheses have been advanced to interpret the disease pathogenesis; however none are able to provide an integrated mechanistic view that can unify the numerous superficially disconnected aspects of AD etiology and pathology. Extracellular amyloid plaques and intracellular neurofibrillary tangles are the two prominent hallmarks of AD neuropathology. It remains unclear what pathogenic events link aggregated proteins such as amyloid beta peptides (Aβ) and/or phosphorylated tau to neuronal damage and death. It is also important to know more precisely how advancing age triggers the disease pathogenesis and how other modifiers affect the disease process. The absence of this basic knowledge is a major barrier not only for understanding of the disease but also for development of effective AD therapies.
Autophagy, or specifically macroautophagy, is a subcellular process participating in membrane trafficking and intracellular degradation and functions in the turnover of damaged organelles and unfavorable proteins through the lysosomal machinery. The autophagy-lysosomal system plays an important role in maintaining intracellular homeostasis and also participates in the pathophysiology of many diseases including cancer, infectious and neurodegenerative diseases (Mizushima et al., 2008). Abnormal autophagic structures have been reported to be extensively involved in AD pathology in brains of human patients as well as animal models (Nixon et al., 2005; Shacka et al., 2008). However, it remains unclear how autophagy contributes to the disease.
Numerous review papers are available that summarize the current knowledge regarding the molecular and cellular aspects of autophagy and its extensive involvement in various diseases. In this chapter, we focus on the concept of an “autophagy-lysosomal cascade” as a key mechanistic insight into AD pathogenesis. This disease hypothesis is based on recent work from our laboratory as well as growing evidence from other AD research groups. The autophagy-lysosomal cascade hypothesis has the capability to integrate many seemingly disconnected aspects of AD pathophysiology into a common cellular framework. We believe that further characterization of the details of autophagic participation in AD will be important for development of anti-Alzheimer’s therapies.
2. Autophagy-derived Alzheimer’s pathogenesis: Signs, lesions and causes
Autophagy-lysosomal involvement in AD and other related animal models has been extensively documented. However, it remains enigmatic if autophagy plays a causative role or is a consequence of the disease process. It is also unclear if autophagy is protective or detrimental with respect to the disease pathogenesis. AD has a multifactorial etiology and also exhibits heterogeneous pathological signs. Correspondingly, numerous disease hypotheses have been proposed primarily based on one or few particular pathological features; currently, no hypothesis can provide a unified mechanistic connection to the hierarchical changes in AD pathogenesis. Practically, an accurate disease mechanism is expected to be attributable to different aspects of the disease etiology and also interpretable to the development of different pathological features of the disease. Here we introduce an autophagy-lysosomal cascade in AD pathogenesis and discuss how this pathogenic cascade is initiated by or contributes to the different aspects of the causes, the signs and the lesions of AD pathophysiology.
2.1. Granulovacuolar degeneration and autophagy-lysosomal neuropathology
Granulovacuolar degeneration (GVD) along with plaques and tangles are the earliest described and also the most prominent histopathologic signs of AD (Anderton, 1997; Ball, 1982; Burger & Vogel, 1973; Funk et al., 2011; Okamoto et al., 1991). Granulovacuolar structures were initially reported for AD in 1911. They are characterized as large translucent vacuoles containing electron-dense granule cores appearing in cytoplasm (Shacka et al., 2008) and are often found in pyramidal neurons of the hippocampus. GVD bodies are double membrane enclosed partially digested cytoplasmic contents (Okamoto et al., 1991), suggesting an autophagic origin for the GVD. This autophagic association is further confirmed by positive immunostaining for LC3 and p62 (autophagic markers), LAMP1 (lysosome-associated membrane protein 1) and CHMP2B (charged multivesicular body protein 2B) to the GVD bodies (Funk et al., 2011; Yamazaki et al., 2010). These studies suggest that the GVD bodies are enlarged vesicles derived from autophagy and endocytosis. GVD may also appear in the normal aging brains where plaques and tangles are sparse (Anderton, 1997).
One of the earliest pathological signs observed in patients with AD is the appearance of numerous enlarged autophagic and endosomal vesicles accumulating in perikarya, neurites and synaptic terminals (Nixon et al., 2005; Nixon et al., 2008; Shacka et al., 2008) due to defective autophagy-lysosomal degradation. The defect was initially thought to result from a putative blockage of vesicle fusion among autophagosomes, endosomes and lysosomes thus leading to the failure for autophagosomes to acquire lysosomal catabolic enzymes necessary for cargo digestion (Boland et al., 2008; Nixon, 2007; Nixon et al., 2005; Yu et al., 2005). This view was primarily based on distinguishing autophagosomes in electron micrographs. The identification of pre- and post-lysosomal autophagic or endosomal vesicles in electron micrographs may be misleading. Distinct types of vesicles can dynamically fuse with each other and thus form diverse highly polymorphic structures. These heterogeneous vesicles are hard to be identified with certainty, especially when compromised as part of the disease process. Failure of lysosomal acidification was also proposed as an alternative mechanism responsible for defective autophagic degradation (Lee et al., 2010). By direct expression of human Aβ1-42 in
Direct Aβ1-42 expression in
2.2. The pathogenic lesions of AD are a result of the autophagy-lysosomal injury
AD exhibits heterogeneous features in its clinical symptoms, histopathology and neurochemistry. Besides the GVD discussed above, other well-documented neuropathological changes include widespread neuron loss, extracellular plaques, intraneuronal tangles, Hirano bodies, defective mitochondria, neurite atrophy, synapse loss, calcium dyshomeostasis, oxidative stress, neuroinflammation, cerebral amyloid angiopathy, etc. The cause-effect relationships between or among these changes have never been clearly established. To clarify the cause-effect relationships among these changes related to neuronal autophagy, we classify them here as pathological signs or pathogenic lesions. A pathological sign is defined as any detectible pathological event not resulting in additional downstream pathological events; whereas a pathogenic lesion is defined as any detectible pathological event causing other downstream pathological events. Previously we proposed a central role of autophagy-lysosomal system in AD pathogenesis (Ling & Salvaterra, 2009). Here we discuss how a primary autophagy-lysosomal injury in neurons might sit at the top of a pathogenic hierarchy and initiate the secondary and tertiary lesions such as mitochondrial dysfunction, oxidative stress, intracellular Ca2+ dyshomeostasis, membrane and organelle damage, all of which eventually develop into the plethora of heterogeneous neuropathologic signs including neurological defects, extracellular diffuse Aβ deposition, amyloid plaques, intracellular tangles, Hirano bodies, neurite and synapse atrophy, extensive neuronal death, etc.
2.2.1. Amyloid deposition and autophagy-lysosomal machinery
A widely held view is that Aβ is produced via APP proteolysis at the surface of neuronal cytoplasmic membranes and released into extracellular spaces. Diffusely distributed extracellular Aβ then assembles into toxic oligomers, aggregates and eventually condenses into senile plaques over a long period of time (Armstrong, 1998; Marchesi, 2005; Torp et al., 2000). However, emerging evidence has demonstrated that a large fraction of Aβ is generated in intracellular compartments rather than at cell surfaces (Gouras et al., 2005; LaFerla et al., 2007). Several subcellular loci have been suggested for intracellular Aβ production including rough endoplasmic reticulum (ER), Golgi apparatus, endosomes, autophagosomes and lysosomes. However, it is unclear how intracellular Aβ is subsequently transported to extracellular spaces and how Aβ deposits into the focal amyloid plaques (Fiala, 2007; Gouras et al., 2005).
The intracellular Aβ may be sequestered by autophagy-lysosomal machinery along with damaged organelles where Aβ is generated. We previously showed that autophagy-sequestered Aβ1-42, in turn, decreases the capacity of autophagy-lysosomal degradation (Ling & Salvaterra, 2011a; Ling et al., 2009). Aβ1-42-induced dysfunction of lysosomal vesicles may retain indigestible Aβ1-42 along with Aβ1-40. In support of this possibility, highly concentrated intracellular Aβ has been identified in various autophagic and endosomal vesicles (Petanceska et al., 2000; Takahashi et al., 2002). Autophagy-lysosomal compartments also function in secretion (Gerasimenko et al., 2001; Griffiths, 2002; Luzio et al., 2007; Manjithaya & Subramani, 2011; Pfeffer, 2010). It is plausible therefore that some of lysosomal vesicles may secret their stored monomeric or oligomeric Aβ peptides into extracellular spaces. Consistent with this, some early observations showed that Aβ is secreted by intracellular secretory compartments (Probst et al., 1991; Rajendran et al., 2006). Highly concentrated Aβ1-42 aggregates stored in enlarged autophagy-lysosomal vesicles may contribute to the development of amyloid plaques during aging or neurodegeneration (our unpublished observation). Thus the neuronal autophagy-lysosomal pathway appears to play a central role in amyloid deposition associated with either AD or normal brain aging.
2.2.2. Lysosome-derived chemical lesions and subcellular damage
Aβ (especially Aβ1-42) is an amphipathic molecule known to disturb biological membranes (Eckert et al., 2010; Gibson Wood et al., 2003). The membranes of lysosome-related vesicles with an acidic microenvironment are especially sensitive to Aβ disturbance (Ditaranto et al., 2001; McLaurin & Chakrabartty, 1996). This membrane disruption is thought to result from the direct interaction between the hydrophobic C-terminus of Aβ peptides and the lipid bilayer of the membrane (Marchesi, 2005). The interaction also appears to be important for membrane-associated Aβ assembly into higher ordered structures (Friedman et al., 2009; Sureshbabu et al., 2010). Compromised membrane integrity greatly increases membrane conductance that has been attributed to a putative ionic channel formed by Aβ peptides (Jang et al., 2010).
Aβ1-42 expressed in
Mitochondrial deficits are a prominent pathogenic lesion in AD (Moreira et al., 2010a; Moreira et al., 2010b; Reddy & Beal, 2008). Lysosomal-derived chemical lesions may be the proximate cause of these deficits. Electron micrographs show a host of morphological changes including decreased size, abnormal cristae and accumulation of osmiophilic materials in brain tissues from AD patients (Baloyannis, 2006). These morphological features are consistent with our observations using the
Lysosomal-derived chemical lesions may also destabilize membranes of ER, nuclei and various transport vesicles that will release Ca2+ into the cytosol. Neuronal Ca2+ is normally stored in membrane compartments such as ER, mitochondria, nuclear envelope and neurotransmitter vesicles (Verkhratsky & Petersen, 1998). Compromise of these membrane-bounded organelles results in a loss of homeostatic intracellular Ca2+ control, another prominent chemical lesion in AD pathogenesis (LaFerla, 2002; Supnet & Bezprozvanny, 2010). Cytoplasmic Ca2+ is a pivotal neuronal signal regulating multiple intraneuronal activities, neural functions and synaptic plasticity. In vitro application of synthetic Aβ can elevate intracellular Ca2+ levels that make cultured neurons more vulnerable to glutamate excitotoxicity (Mattson et al., 1992). Disturbances in neuronal Ca2+ may also affect mitochondrial function and vesicular trafficking and, in turn, exacerbate the neurodegenerative cascade.
Lysosomal-derived chemical lesions can destabilize the cytoskeleton, a subcellular component essential for axonal transport, maintenance of normal structure and function of neurites and synapses as well as other cellular activities. Elevated intracellular Ca2+ alone was observed to be sufficient to destabilize microtubules and accelerate tau phosphorylation (Mattson et al., 1991), thus linking this chemical lesion with the formation of neurofibrillary tangles. Lysosomal-derived chemical lesions are also associated with the formation of Hirano bodies, rod-shaped and paracrystalline intracellular aggregates composed of actin and cofilin (Maciver & Harrington, 1995). Many neurodegenerative conditions induce the rapid formation of cofilin-actin rod-like inclusions that occur primarily in axons and neurites (Minamide et al., 2000). Cytoskeletal destabilization will disrupt axonal transport of mitochondria and neurotransmitter vesicles as well as many other important subcellular activities in neurons (McMurray, 2000; Stokin et al., 2005). Tau hyperphosphorylation and microtubule destabilization will also accelerate neurite and synapse atrophy due to the crucial role of microtubules in supporting neuronal terminals and maintaining synaptic integrity (Harada et al., 1994). Thus lysosomal-derived chemical lesions may initiate multiple downstream pathogenic lesions via oxidative stress, Ca2+ aberration, cytoplasmic acidification, etc leading to a self-exacerbating and vicious cycle.
Membrane integrity is essential for implementation of neuronal function because the conduction of nerve impulses depends on the maintenance of stable ionic gradients. After an electrical signaling event, restoration of active membrane properties requires an intact membrane to restore proper ionic gradients. Normal neuronal function also relies on the integrity of neurites that extend far from cell bodies. Thus the abnormally elevated Ca2+ levels, destabilized microtubules and other cytoskeletal elements, defects in axonal transport, degenerating neurites and synapses resulting from lysosome-derived chemical lesions will cause a decline in neuronal functional performance that may contribute to impairment in the encoding or retrieval of new memories, one of the earliest signs of AD (Selkoe, 2002).
2.2.3. Autophagy-lysosomal injury contributes to neurite and synapse atrophy
Alzheimer’s dementia is believed to start from synaptic alterations that correlate more robustly with cognitive decline, memory loss and neurodegeneration than the traditional pathological markers such as plaques and tangles (Selkoe, 2002). Synapse loss and neurite atrophy is critically dependent on cortical Aβ levels. Direct expression of Aβ in
2.2.4. Widespread neuronal loss and autophagy-derived necrosis
A major unanswered question in Alzheimer’s pathogenesis is to identify the execution pathway responsible for widespread neuronal death. Apoptosis, a well-controlled and self-regulated programmed cell death, has been widely considered to be the relevant cell death mechanism in many neurodegenerative disorders. However, this appealing mechanism is problematic when applied to Alzheimer’s pathogenesis (Graeber & Moran, 2002). Apoptosis is characterized by DNA fragmentation, chromatin condensation, caspase activation, cell shrinking and plasma membrane blebbing. DNA fragmentation detected by the TUNEL method is widespread in AD type neuronal death; however apoptotic morphology is rare (Jellinger & Stadelmann, 2000). DNA fragmentation, phosphatidylserine exposure on the cell surface as well as mitochondrial dysfunction also exist in other non-apoptotic types of cell death, raising the concern that the widely used TUNEL or annexin V staining alone is not sufficient to validate apoptosis as a particular cell death mechanism.
Autophagy, while generally viewed as a cell survival mechanism, is also thought to cause autophagic cell death (Bursch, 2001), another type of programmed cell death characterized by an abundance of autophagic vesicles in dying cells (Chen et al., 2010). Autophagy over-activation in
Either brain aging or Aβ1-42 production causes a chronic deterioration of the neuronal autophagy-lysosomal system leading to accumulation of inefficient and enlarged autophagy-lysosomal vesicles in neurons (Ling & Salvaterra, 2011a). Lysosomal compartments are known for membrane permeabilization that release lysosomal cathepsins and other hydrolases into the cytosol; however, the process and the extent of the leakage are usually regulable or may activate a controlled mode of cell death (i.e. apoptosis) (Boya & Kroemer, 2008; Guicciardi et al., 2004). Intriguingly, we found that Aβ1-42-induced lysosomal leakage causes uncontrollable intraneuronal necrotic destruction (Ling et al., 2009). Some dying neurons lose their normal cytosolic structures but maintain a relatively normal shape for the plasma membrane forming balloon cells (Fig. 3). Necrotic cell death usually stimulates a powerful inflammatory response. Indeed, neuroinflammation is a prominent pathological feature of AD (Sastre et al., 2011). These data indicate that autophagy-derived necrosis is likely to be the primary cell death execution pathway responsible for the widespread neuronal loss in AD pathogenesis.
2.3. Causative connections between AD risk factors and autophagy-lysosomal injury
The firmly established risk factors of AD are increasing age, the ε4 allele of the apolipoprotein E (
2.3.1. Genetic determinants and autophagy-lysosomal Aβ degradation
Amyloid deposition formed by Aβ aggregates is a pathological hallmark of AD. Familial AD-associated mutations on
Aβ1–42-induced neurodegeneration via an autophagy-lysosomal injury does not conflict with the general protective function of the autophagy-lysosomal machinery. The protective or detrimental effect of neuronal autophagy is primarily dependent on the efficiency of lysosomal degradation of disease-associated aggregate-prone proteins and damaged organelles. Not all aggregate-prone proteins are amenable to autophagic degradation (Wong et al., 2008). Human Aβ1–40 and Aβ1–42 expressed in
2.3.2. The risk factors of ApoE and cholesterol
ApoE and cholesterol, known to have a strong impact on development of cardiovascular disease (Purnell et al., 2009), are also important modifiers of AD onset (Lahiri et al., 2004; Sambamurti et al., 2004). The underlying mechanism linking ApoE and cholesterol with AD pathogenesis is still not completely understood. Cholesterol is a normal membrane component that modifies membrane fluidity. Accumulating evidence shows that cholesterol modulates Aβ production and aggregation through its effect on lipid rafts. Membrane-embedded APP undergoes amyloidogenic proteolysis by beta-secretase (BACE1) or non-amyloidogenic proteolysis by alpha-secretase. Lipid rafts, the cholesterol- and sphingolipid-enriched membrane microdomains (Simons & Toomre, 2000), play an essential role in amyloidogenic APP proteolysis, because the lipid raft enhances accessibility of BACE1 to APP (Ehehalt et al., 2003; Rushworth & Hooper, 2010; Vetrivel & Thinakaran, 2010). Lipid rafts may also facilitate Aβ aggregation (Rushworth & Hooper, 2010) and extracellular Aβ internalization (Lai & McLaurin, 2010). Increased cholesterol accelerates APP localization into lipid rafts and enhances Aβ generation (Kosicek et al., 2010; Michikawa, 2003); consistent with observations that elevated dietary cholesterol uptake or hypercholesterolemia is associated with increased formation of amyloid plaques (Kivipelto et al., 2001). In addition, cholesterol depletion inhibits neuronal Aβ generation (Sambamurti et al., 2004); and cholesterol-reducing statin drugs appear to reduce the risk of dementia (Gibson Wood et al., 2003).
ApoE is the major carrier of lipids, including cholesterol, in the brain. Lipidated ApoE has been shown to inhibit Aβ transport across blood-brain-barrier and facilitate its degradation (Fan et al., 2009). The ε4 allele of
2.3.3. Brain aging and autophagy-lysosomal catabolism
AD exhibits multiple neuropathological signs and clinical symptoms that distinguish it from normal brain aging. However, normal aging brains undergo similar histopathologic changes seen in AD including the presence of plaques, tangles, Hirano bodies, GVD, neurite and synapse deficit, shrinkage in overall brain volume, decreased brain weight and enlargement of brain ventricles (Anderton, 1997; Drachman, 2007). The differences in these changes comparing AD with normal aging appear to be quantitative rather than qualitative (Ball, 1982). Even after a century of intensive studies, the pathogenic connection between normal aging and AD remains elusive.
Human Aβ1-42 expression in
Autophagy-lysosomal machinery maintains intracellular homeostasis and thus protects neurons from degeneration. Basal levels of neuronal autophagy are believed to decrease with age (Komatsu et al., 2007); however, direct evidence supporting this view is absent. In
3. Autophagy-lysosomal cascade: A hypothesis for AD pathogenesis
Remarkable progress has been made in studying many aspects of AD. Unfortunately, this has not resulted in the successful development of effective treatments, primarily because of the absence of a definite pathogenic mechanism. Numerous hypotheses have been advanced to address AD pathogenesis including the amyloid cascade, membrane disruption/Aβ ion channel, mitochondrial abnormalities, energy deficits, glutamate excitotoxicity, cerebrovascular dysfunction, neuroinflammation, oxidative stress, Ca2+ dyshomeostasis and cytoskeletal aberrations. Each of these ideas were proposed and developed based on one or few particular pathological features of AD. As a consequence most of the currently favored hypotheses provide only a limited view rather than a more global perspective of the pathogenic mechanism. It also remains unclear what initial event(s) trigger the pathogenic cascade and how so many different pathological insults can be attributed to the key pathogenic event.
Extensive autophagy involvement in AD has been well documented (Nixon et al., 2005; Shacka et al., 2008; Suzuki & Terry, 1967). However, it remains unsettled if autophagy plays a causative role, a protective role or is a consequence of the disease process itself (Ling & Salvaterra, 2009). Among the various signs and lesions of AD neuropathology, compromised autophagy-lysosomal vesicles and their resultant injuries appear to play a central role in initiating the pathogenic cascade leading to disease progression. Based on
APP proteolysis and Aβ production occurs at membrane surfaces facing the lumen of membrane compartments including ER, Golgi apparatus and endosomal vesicles (Fiala, 2007; Gouras et al., 2005). Aβ is constitutively produced in human brains throughout the normal lifespan. Apparently the levels of newly generated Aβ peptides may not be sufficient to initiate a pathogenic cascade in healthy neurons; however, due to their amphipathic property, they may disturb local membranes and the functional execution of host organelles. These organelles, if damaged, will be sequestered by autophagy. However, Aβ particularly Aβ1-42 cannot be efficiently degraded in autophagy-lysosomal vesicles especially under chronic deterioration of this machinery during advancing age (Ling & Salvaterra, 2011a). Other indigestible proteins and lipids (for example lipofuscin) may synergistically contribute to the deterioration of autophagy-lysosomal machinery causing cargo storage in enlarged vesicles. Consistent with this view, intracellular Aβ peptides predominantly accumulate within autophagic and endosomal vesicles (Nixon, 2004; Takahashi et al., 2002; Yu et al., 2005) and AD-like neuropathological phenotypes are also seen in some lysosomal storage diseases (Bahr & Bendiske, 2002; Ohm et al., 2003; Jin et al., 2004; Settembre et al., 2008).
Numerous lysosomal vesicles in cytosol would represent a large source of acidic contents and lysosomal hydrolases. The enlarged size and long-term retention of these vesicles may make them easily compromised especially when Aβ1-42 becomes concentrated within them. Compromised vesicles result in leakage of their acidic contents into cytosol. This will destabilize other intracellular structures and organelles including ER and mitochondria leading to oxidative stress and Ca2+ dyshomeostasis. The resultant damage from this altered intracellular microenvironment will further activate autophagy causing additional pathogenic stress. Thus a self-exacerbated pathogenic cascade is formed through initiation, dysfunction, compromise of autophagic vesicles and the resultant cytosolic chemical lesions. This neurodegenerative cascade is initiated by Aβ1-42 and enhanced by aging and eventually results in necrotic neuronal death. Once initiated, the cascade would likely become independent of continuous Aβ production since cytosolic chemical lesions would drive it as a progressive and irreversible pathogenic pathway. This autophagy-lysosomal-derived neurodegenerative cascade provides a common cellular framework for a detailed mechanistic understanding of the heterogeneous aspects of AD neuropathology as the signs, the lesions and the causes of the disease.
Alzheimer’s disease is an incurable terminal neurodegenerative disorder with multifactorial etiology and heterogeneous pathology. The clearer we understand the pathogenic mechanism(s) regarding its causes, lesions and signs, the better we should be able to develop effective treatments for mitigating or even preventing this disastrous disorder. The autophagy-lysosomal system, a bulk process for removal of intracellular toxic proteins and damaged organelles, appears to play a central role in the disease pathogenesis. Based on our recent work and a large volume of previous studies from other groups, we propose an autophagy-lysosomal cascade that is attributable to various AD etiologies, and responsible for the hierarchical pathological signs and pathogenic lesions. One of the prominent features of this pathogenic mechanism is its potential for self-exacerbation. Once progressing to an uncontrollable stage, this cascade is likely to be independent of initial contributions from causative factors and will continue to develop progressively and irreversibly. This feature fits well with the onset of pathological and clinical AD. It has never been clear when the disease pathology actually starts; however, once diagnosed, the disease develops progressively and relentlessly. This feature emphasizes the importance of preventative strategies applied to the at-risk individuals prior to the actual occurrence of this disease.
The autophagy-lysosomal cascade for AD pathogenesis appears to provide a unified cellular framework for understanding the disease; however, therapeutic development targeting autophagy-lysosomal pathway is far from maturation. Our knowledge of the autophagy-lysosomal system is fast growing (Klionsky, 2007). Many basic aspects of the pathway are still waiting for detailed characterization. A beneficial outcome from manipulation of autophagy activity under neurodegenerative conditions is still uncertain. Even though basal autophagy is protective and autophagy induction has prosurvival effects observed in some disease models (Rubinsztein et al., 2007), detrimental effects of increased autophagy are also associated with certain pathological conditions (Cherra et al., 2010; White & DiPaola, 2009). Our studies, however, emphasize that enhancing the maintenance of an integrated and efficient autophagy–lysosomal system in brain rather than simply induction of autophagy activity would be a promising therapeutic direction for anti-aging or prevention of AD.
The work in our laboratory has been generously supported by grants from the American Health Assistant Foundation (AHAF) and the Sidell-Kagan Foundation. DL was partially supported by postdoctoral fellowships from the John Douglas French Alzheimer's Foundation (2005-2007) and the American Federation for Aging Research (2009-2010). The authors thank all other authors making contributions to the studies cited in this manuscript and apologize to those who made similar contributions in work not being cited due to space limitation. The authors have no competing financial interests.
Anderton B. H. 1997Changes in the ageing brain in health and disease. Philosophical transactions of the Royal Society of London 352 1781 1792
Armstrong R. A. 1998Beta-amyloid plaques: stages in life history or independent origin? Dementia and geriatric cognitive disorders 9 227 238
Bahr B. A. Bendiske J. 2002The neuropathogenic contributions of lysosomal dysfunction. Journal of neurochemistry 83 481 489
Balaban R. S. Nemoto S. Finkel T. 2005Mitochondria, oxidants, and aging. Cell 120 483 495
Ball M. J. 1982Alzheimer’s disease: a challenging enigma. Archives of pathology & laboratory medicine 106 157 162
Ball M. J. Lo P. 1977Granulovacuolar degeneration in the ageing brain and in dementia. Journal of neuropathology and experimental neurology 36 474 487
Baloyannis S. J. 2006Mitochondrial alterations in Alzheimer’s disease. Journal of Alzheimer’s disease 9 119 126
Bergamini E. Cavallini G. Donati A. Gori Z. 2007The role of autophagy in aging: its essential part in the anti-aging mechanism of caloric restriction. Annals of the New York Academy of Sciences 1114 69 78
Bertram L. Tanzi R. E. 2008Thirty years of Alzheimer’s disease genetics: the implications of systematic meta-analyses.
Boland B. Kumar A. Lee S. Platt F. M. Wegiel J. Yu W. H. Nixon R. A. 2008Autophagy induction and autophagosome clearance in neurons: relationship to autophagic pathology in Alzheimer’s disease. Journal of Neuroscience 28 6926 6937
Boya P. Kroemer G. 2008Lysosomal membrane permeabilization in cell death. Oncogene 27 6434 6451
Burger P. C. Vogel F. S. 1973The development of the pathologic changes of Alzheimer’s disease and senile dementia in patients with Down’s syndrome. The American journal of pathology 73 457 476
Bursch W. 2001The autophagosomal-lysosomal compartment in programmed cell death. Cell death and differentiation 8 569 581
Cataldo A. M. Peterhoff C. M. Troncoso J. C. Gomez-Isla T. Hyman B. T. Nixon R. A. 2000Endocytic pathway abnormalities precede amyloid beta deposition in sporadic Alzheimer’s disease and Down syndrome: differential effects of APOE genotype and presenilin mutations. The American journal of pathology 157 277 286
Chen Y. Azad M. B. Gibson S. B. 2010Methods for detecting autophagy and determining autophagy-induced cell death. Canadian journal of physiology and pharmacology 88 285 295
Cherra S. J. 3rd Dagda R. K. Chu C. T. 2010Autophagy and neurodegeneration: survival at a cost? Neuropathology and applied neurobiology 36 125 132
Chu C. T. Plowey E. D. Dagda R. K. Hickey R. W. Cherra S. J. 3rd Clark R. S. 2009Autophagy in neurite injury and neurodegeneration: in vitro and in vivo models. Methods in enzymology 453 217 249
Chu C. T. Zhu J. Dagda R. 2007Beclin 1-independent pathway of damage-induced mitophagy and autophagic stress: implications for neurodegeneration and cell death. Autophagy 3 663 666
Crowther D. C. Kinghorn K. J. Miranda E. Page R. Curry J. A. Duthie F. A. Gubb D. C. Lomas D. A. 2005Intraneuronal Abeta, non-amyloid aggregates and neurodegeneration in a
De Kroon R. M. Armati P. J. 2001The endosomal trafficking of apolipoprotein E3 and E4 in cultured human brain neurons and astrocytes. Neurobiology of disease 8 78 89
Ditaranto K. Tekirian T. L. Yang A. J. 2001Lysosomal membrane damage in soluble Abeta-mediated cell death in Alzheimer’s disease. Neurobiology of disease 8 19 31
Drachman D. A. 2007Rethinking Alzheimer’s disease: the role of age-related changes. Current neurology and neuroscience reports 7 265 268
Eckert G. P. Wood W. G. Muller W. E. 2010Lipid membranes and beta-amyloid: a harmful connection. Current protein & peptide science 11 319 325
Ehehalt R. Keller P. Haass C. Thiele C. Simons K. 2003Amyloidogenic processing of the Alzheimer beta-amyloid precursor protein depends on lipid rafts. The Journal of cell biology 160 113 123
Fan J. Donkin J. Wellington C. 2009Greasing the wheels of Abeta clearance in Alzheimer’s disease: the role of lipids and apolipoprotein E. BioFactors (Oxford, England) 35 239 248
Fiala J. C. 2007Mechanisms of amyloid plaque pathogenesis. Acta neuropathologica 114 551 571
Finelli A. Kelkar A. Song H. J. Yang H. Konsolaki M. 2004A model for studying Alzheimer’s Abeta42-induced toxicity in Drosophila melanogaster.
Flicker L. 2010Modifiable lifestyle risk factors for Alzheimer’s disease. Journal of Alzheimer’s disease 20 803 811
Friedman R. Pellarin R. Caflisch A. 2009Amyloid aggregation on lipid bilayers and its impact on membrane permeability. Journal of molecular biology 387 407 415
Funk K. E. Mrak R. E. Kuret J. 2011Granulovacuolar Degeneration Bodies of Alzheimer’s Disease Resemble Late-stage Autophagic Organelles.
Gerasimenko J. V. Gerasimenko O. V. Petersen O. H. 2001Membrane repair: Ca(2+)-elicited lysosomal exocytosis.
Gibson Wood. W. Eckert G. P. Igbavboa U. Muller W. E. 2003Amyloid beta-protein interactions with membranes and cholesterol: causes or casualties of Alzheimer’s disease. Biochimica et biophysica acta 1610 281 290
Gilman S. Koller M. Black R. S. Jenkins L. Griffith S. G. Fox N. C. Eisner L. Kirby L. Rovira M. B. Forette F. et al. 2005Clinical effects of Abeta immunization (AN1792) in patients with AD in an interrupted trial. Neurology 64 1553 1562
Gouras G. K. Almeida C. G. Takahashi R. H. 2005Intraneuronal Abeta accumulation and origin of plaques in Alzheimer’s disease. Neurobiology of aging 26 1235 1244
Graeber M. B. Moran L. B. 2002Mechanisms of cell death in neurodegenerative diseases: fashion, fiction, and facts.
Griffiths G. 2002What’s special about secretory lysosomes? Seminars in cell & developmental biology 13 279 284
Guicciardi M. E. Leist M. Gores G. J. 2004Lysosomes in cell death. Oncogene 23 2881 2890
Harada A. Oguchi K. Okabe S. Kuno J. Terada S. Ohshima T. Sato-Yoshitake R. Takei Y. Noda T. Hirokawa N. 1994Altered microtubule organization in small-calibre axons of mice lacking tau protein. Nature 369 488 491
Hartmann T. Bieger S. C. Bruhl B. Tienari P. J. Ida N. Allsop D. Roberts G. W. Masters C. L. Dotti C. G. Unsicker K. et al. 1997Distinct sites of intracellular production for Alzheimer’s disease A beta40/42 amyloid peptides. Nature medicine 3 1016 1020
Iijima K. Liu H. P. Chiang A. S. Hearn S. A. Konsolaki M. et al. 2004Dissecting the pathological effects of human Abeta40 and Abeta42 in Drosophila: a potential model for Alzheimer’s disease.
Jang H. Arce F. T. Ramachandran S. Capone R. Azimova R. Kagan B. L. Nussinov R. Lal R. 2010Truncated beta-amyloid peptide channels provide an alternative mechanism for Alzheimer’s Disease and Down syndrome. Proceedings of the National Academy of Sciences of the United States of America 107 6538 6543
Jellinger K. A. Stadelmann C. 2000Mechanisms of cell death in neurodegenerative disorders. Journal of neural transmission 59 95 114
Jin L. W. Shie F. S. Maezawa I. Vincent I. Bird T. 2004Intracellular accumulation of amyloidogenic fragments of amyloid-beta precursor protein in neurons with Niemann-Pick type C defects is associated with endosomal abnormalities.
Jones P. B. Adams K. W. Rozkalne A. Spires-Jones T. L. Hshieh T. T. Hashimoto T. von Armin. C. A. Mielke M. Bacskai B. J. Hyman B. T. 2011Apolipoprotein E: Isoform Specific Differences in Tertiary Structure and Interaction with Amyloid-beta in Human Alzheimer Brain.
Kivipelto M. Helkala E. L. Laakso M. P. Hanninen T. Hallikainen M. Alhainen K. Soininen H. Tuomilehto J. Nissinen A. 2001Midlife vascular risk factors and Alzheimer’s disease in later life: longitudinal, population based study.
Klionsky D. J. 2007Autophagy: from phenomenology to molecular understanding in less than a decade. Nature reviews 8 931 937
Knoferle J. Koch J. C. Ostendorf T. Michel U. Planchamp V. Vutova P. Tonges L. Stadelmann C. Bruck W. Bahr M. et al. 2010Mechanisms of acute axonal degeneration in the optic nerve in vivo. Proceedings of the National Academy of Sciences of the United States of America 107 6064 6069
Koike M. Shibata M. Tadakoshi M. Gotoh K. Komatsu M. Waguri S. Kawahara N. Kuida K. Nagata S. Kominami E. et al. 2008Inhibition of autophagy prevents hippocampal pyramidal neuron death after hypoxic-ischemic injury. The American journal of pathology 172 454 469
Komatsu M. Ueno T. Waguri S. Uchiyama Y. Kominami E. Tanaka K. 2007Constitutive autophagy: vital role in clearance of unfavorable proteins in neurons. Cell death and differentiation 14 887 894
Kosicek M. Malnar M. Goate A. Hecimovic S. 2010Cholesterol accumulation in Niemann Pick type C (NPC) model cells causes a shift in APP localization to lipid rafts. Biochemical and biophysical research communications 393 404 409
Kumar-Singh S. 2008Cerebral amyloid angiopathy: pathogenetic mechanisms and link to dense amyloid plaques.
La Ferla F. M. 2002Calcium dyshomeostasis and intracellular signalling in Alzheimer’s disease. Nature Review Neuroscience 3 862 872
La Ferla F. M. Green K. N. Oddo S. 2007Intracellular amyloid-beta in Alzheimer’s disease.
Lahiri D. K. Maloney B. 2010The "LEARn" (Latent Early-life Associated Regulation) model integrates environmental risk factors and the developmental basis of Alzheimer’s disease, and proposes remedial steps. Experimental gerontology 45 291 296
Lahiri D. K. Sambamurti K. Bennett D. A. 2004Apolipoprotein gene and its interaction with the environmentally driven risk factors: molecular, genetic and epidemiological studies of Alzheimer’s disease. Neurobiology of aging 25 651 660
Lai A. Y. Mc Laurin J. 2010Mechanisms of amyloid-Beta Peptide uptake by neurons: the role of lipid rafts and lipid raft-associated proteins.
Lee J. H. Yu W. H. Kumar A. Lee S. Mohan P. S. Peterhoff C. M. Wolfe D. M. Martinez-Vicente M. Massey A. C. Sovak G. et al. 2010Lysosomal proteolysis and autophagy require presenilin 1 and are disrupted by Alzheimer-related PS1 mutations. Cell 141 1146 1158
Ling D. Salvaterra P. M. 2009A central role for autophagy in Alzheimer-type neurodegeneration. Autophagy 5 738 740
Ling D. Salvaterra P. M. 2011aBrain aging and Abeta (1-42) neurotoxicity converge via deterioration in autophagy-lysosomal system: a conditional
Ling D. Salvaterra P. M. 2011bRobust RT-qPCR data normalization: validation and selection of internal reference genes during post-experimental data analysis.
Ling D. Song H. J. Garza D. Neufeld T. P. Salvaterra P. M. 2009Abeta 42induced neurodegeneration via an age-dependent autophagic-lysosomal injury in
Luzio J. P. Pryor P. R. Bright N. A. 2007Lysosomes: fusion and function. Nature reviews 8 622 632
Maciver S. K. Harrington C. R. 1995Two actin binding proteins, actin depolymerizing factor and cofilin, are associated with Hirano bodies. Neuroreport 6 1985 1988
Manjithaya R. Subramani S. 2011Autophagy: a broad role in unconventional protein secretion? Trends in cell biology 21 67 73
Marchesi V. T. 2005An alternative interpretation of the amyloid Abeta hypothesis with regard to the pathogenesis of Alzheimer’s disease. Proceedings of the National Academy of Sciences of the United States of America 102 9093 9098
Martins I. J. Hone E. Foster J. K. Sunram-Lea S. I. Gnjec A. Fuller S. J. Nolan D. Gandy S. E. Martins R. N. 2006Apolipoprotein E, cholesterol metabolism, diabetes, and the convergence of risk factors for Alzheimer’s disease and cardiovascular disease.
Mattson M. P. Cheng B. Davis D. Bryant K. Lieberburg I. Rydel R. E. 1992Beta-Amyloid peptides destabilize calcium homeostasis and render human cortical neurons vulnerable to excitotoxicity. Journal of Neuroscience 12 376 389
Mattson M. P. Engle M. G. Rychlik B. 1991Effects of elevated intracellular calcium levels on the cytoskeleton and tau in cultured human cortical neurons.
Mc Dowell I. 2001Alzheimer’s disease: insights from epidemiology. Aging (Milan, Italy) 13 143 162
Mc Gowan E. Eriksen J. Hutton M. 2006A decade of modeling Alzheimer’s disease in transgenic mice. Trends in Genetics 22 281 289
Mc Laurin J. Chakrabartty A. 1996Membrane disruption by Alzheimer beta-amyloid peptides mediated through specific binding to either phospholipids or gangliosides. Implications for neurotoxicity. The Journal of biological chemistry 271 26482 26489
Mc Murray C. T. 2000Neurodegeneration: diseases of the cytoskeleton? Cell death and differentiation 7 861 865
Michikawa M. 2003The role of cholesterol in pathogenesis of Alzheimer’s disease: dual metabolic interaction between amyloid beta-protein and cholesterol. Molecular neurobiology 27 1 12
Minamide L. S. Striegl A. M. Boyle J. A. Meberg P. J. Bamburg J. R. 2000Neurodegenerative stimuli induce persistent ADF/cofilin-actin rods that disrupt distal neurite function. Nature cell biology 2 628 636
Mizushima N. Levine B. Cuervo A. M. Klionsky D. J. 2008Autophagy fights disease through cellular self-digestion. Nature 451 1069 1075
Moreira P. I. Carvalho C. Zhu X. Smith M. A. Perry G. 2010aMitochondrial dysfunction is a trigger of Alzheimer’s disease pathophysiology. Biochimica et biophysica acta 1802 2 10
Moreira P. I. Santos R. X. Zhu X. Lee H. G. Smith M. A. Casadesus G. Perry G. 2010bAutophagy in Alzheimer’s disease. Expert review of neurotherapeutics 10 1209 1218
Nixon R. A. 2004Niemann-Pick Type C disease and Alzheimer’s disease: the APP-endosome connection fattens up. The American journal of pathology 164 757 761
Nixon R. A. 2007Autophagy, amyloidogenesis and Alzheimer disease. Journal of cell science 120 4081 4091
Nixon R. A. Wegiel J. Kumar A. Yu W. H. Peterhoff C. Cataldo A. Cuervo A. M. 2005Extensive involvement of autophagy in Alzheimer disease: an immuno-electron microscopy study. Journal of neuropathology and experimental neurology 64 113 122
Nixon R. A. Yang D. S. Lee J. H. 2008Neurodegenerative lysosomal disorders: a continuum from development to late age. Autophagy 4 590 599
Ohm T. G. Treiber-Held S. Distl R. Glockner F. Schonheit B. Tamanai M. Meske V. 2003Cholesterol and tau protein--findings in Alzheimer’s and Niemann Pick C’s disease.
Okamoto K. Hirai S. Iizuka T. Yanagisawa T. Watanabe M. 1991Reexamination of granulovacuolar degeneration. Acta neuropathologica 82 340 345
Petanceska S. S. Seeger M. Checler F. Gandy S. 2000Mutant presenilin 1 increases the levels of Alzheimer amyloid beta-peptide Abeta42 in late compartments of the constitutive secretory pathway. Journal of neurochemistry 74 1878 1884
Pfeffer S. R. 2010Unconventional secretion by autophagosome exocytosis. The Journal of cell biology 188 451 452
Probst A. Langui D. Ipsen S. Robakis N. Ulrich J. 1991Deposition of beta/A4 protein along neuronal plasma membranes in diffuse senile plaques. Acta neuropathologica 83 21 29
Purnell C. Gao S. Callahan C. M. Hendrie H. C. 2009Cardiovascular risk factors and incident Alzheimer disease: a systematic review of the literature. Alzheimer disease and associated disorders 23 1 10
Raber J. Huang Y. Ashford J. W. 2004ApoE genotype accounts for the vast majority of AD risk and AD pathology. Neurobiology of aging 25 641 650
Rajendran L. Honsho M. Zahn T. R. Keller P. Geiger K. D. Verkade P. Simons K. 2006Alzheimer’s disease beta-amyloid peptides are released in association with exosomes. Proceedings of the National Academy of Sciences of the United States of America 103 11172 11177
Reddy P. H. Beal M. F. 2008Amyloid beta, mitochondrial dysfunction and synaptic damage: implications for cognitive decline in aging and Alzheimer’s disease. Trends in molecular medicine 14 45 53
Rensen P. C. Jong M. C. van Vark L. C. van der Boom H. Hendriks W. L. van Berkel T. J. Biessen E. A. Havekes L. M. 2000Apolipoprotein E is resistant to intracellular degradation in vitro and in vivo. Evidence for retroendocytosis. The Journal of biological chemistry 275 8564 8571
Rosendorff C. Beeri M. S. Silverman J. M. 2007Cardiovascular risk factors for Alzheimer’s disease. The American journal of geriatric cardiology 16 143 149
Roses A. D. 1996Apolipoprotein E alleles as risk factors in Alzheimer’s disease. Annual review of medicine 47 387 400
Rowland A. M. Richmond J. E. Olsen J. G. Hall D. H. Bamber B. A. 2006Presynaptic terminals independently regulate synaptic clustering and autophagy of GABAA receptors in Caenorhabditis elegans. Journal of Neuroscience 26 1711 1720
Rubinsztein D. C. Gestwicki J. E. Murphy L. O. Klionsky D. J. 2007Potential therapeutic applications of autophagy.
Rushworth J. V. Hooper N. M. 2010Lipid Rafts: Linking Alzheimer’s Amyloid-beta Production, Aggregation, and Toxicity at Neuronal Membranes.
Sambamurti K. Granholm A. C. Kindy M. S. Bhat N. R. Greig N. H. Lahiri D. K. Mintzer J. E. 2004Cholesterol and Alzheimer’s disease: clinical and experimental models suggest interactions of different genetic, dietary and environmental risk factors. Current drug targets 5 517 528
Sastre M. Richardson J. C. Gentleman S. M. Brooks D. J. 2011Inflammatory Risk Factors and Pathologies Associated with Alzheimer’s Disease.
Scott R. C. Juhasz G. Neufeld T. P. 2007Direct induction of autophagy by Atg1 inhibits cell growth and induces apoptotic cell death. Current Biology 17 1 11
Selkoe D. J. 2002Alzheimer’s disease is a synaptic failure. Science 298 789 791
Settembre C. Fraldi A. Jahreiss L. Spampanato C. Venturi C. Medina D. de Pablo R. Tacchetti C. Rubinsztein D. C. Ballabio A. 2008A block of autophagy in lysosomal storage disorders. Human molecular genetics 17 119 129
Shacka J. J. Roth K. A. Zhang J. 2008The autophagy-lysosomal degradation pathway: role in neurodegenerative disease and therapy. Frontiers in bioscience 13 718 736
Simons K. Toomre D. 2000Lipid rafts and signal transduction. Nature reviews 1 31 39
Sleegers K. Lambert J. C. Bertram L. Cruts M. Amouyel P. Van Broeckhoven C. 2010The pursuit of susceptibility genes for Alzheimer’s disease: progress and prospects. Trends in Genetics 26 84 93
Stokin G. B. Lillo C. Falzone T. L. Brusch R. G. Rockenstein E. Mount S. L. Raman R. Davies P. Masliah E. Williams D. S. et al. 2005Axonopathy and transport deficits early in the pathogenesis of Alzheimer’s disease. Science 307 1282 1288
Supnet C. Bezprozvanny I. 2010Neuronal calcium signaling, mitochondrial dysfunction, and Alzheimer’s disease.
Sureshbabu N. Kirubagaran R. Thangarajah H. Malar E. J. Jayakumar R. 2010Lipid-induced conformational transition of amyloid beta peptide fragments. Journal of Molecular Neuroscience 41 368 382
Suzuki K. Terry R. D. 1967Fine structural localization of acid phosphatase in senile plaques in Alzheimer’s presenile dementia. Acta neuropathologica 8 276 284
Takahashi R. H. Milner T. A. Li F. Nam E. E. Edgar M. A. Yamaguchi H. Beal M. F. Xu H. Greengard P. Gouras G. K. 2002Intraneuronal Alzheimer abeta42 accumulates in multivesicular bodies and is associated with synaptic pathology. The American journal of pathology 161 1869 1879
Torp R. Head E. Milgram N. W. Hahn F. Ottersen O. P. Cotman C. W. 2000Ultrastructural evidence of fibrillar beta-amyloid associated with neuronal membranes in behaviorally characterized aged dog brains. Neuroscience 96 495 506
Verkhratsky A. J. Petersen O. H. 1998Neuronal calcium stores. Cell calcium 24 333 343
Vetrivel K. S. Thinakaran G. 2010Membrane rafts in Alzheimer’s disease beta-amyloid production. Biochimica et biophysica acta 1801 860 867
Wang Q. J. Ding Y. Kohtz D. S. Mizushima N. Cristea I. M. Rout M. P. Chait B. T. Zhong Y. Heintz N. Yue Z. 2006Induction of autophagy in axonal dystrophy and degeneration. Journal of Neuroscience 26 8057 8068
White E. Di Paola R. S. 2009The double-edged sword of autophagy modulation in cancer. Clinical Cancer Research 15 5308 5316
Wong E. S. Tan J. M. Soong W. E. Hussein K. Nukina N. Dawson V. L. Dawson T. M. Cuervo A. M. Lim K. L. 2008Autophagy-mediated clearance of aggresomes is not a universal phenomenon. Human molecular genetics 17 2570 2582
Yamauchi K. Tozuka M. Hidaka H. Nakabayashi T. Sugano M. Katsuyama T. 2002Isoform-specific effect of apolipoprotein E on endocytosis of beta-amyloid in cultures of neuroblastoma cells. Annals of clinical and laboratory science 32 65 74
Yamazaki Y. Takahashi T. Hiji M. Kurashige T. Izumi Y. Yamawaki T. Matsumoto M. 2010Immunopositivity for ESCRT-III subunit CHMP2B in granulovacuolar degeneration of neurons in the Alzheimer’s disease hippocampus. Neuroscience letters 477 86 90
Yang Y. Fukui K. Koike T. Zheng X. 2007Induction of autophagy in neurite degeneration of mouse superior cervical ganglion neurons. The European journal of neuroscience 26 2979 2988
Yu C. Nwabuisi-Heath E. Laxton K. Ladu M. J. 2010Endocytic pathways mediating oligomeric Abeta42 neurotoxicity.
Yu W. H. Cuervo A. M. Kumar A. Peterhoff C. M. Schmidt S. D. Lee J. H. Mohan P. S. Mercken M. Farmery M. R. Tjernberg L. O. et al. 2005Macroautophagy--a novel Beta-amyloid peptide-generating pathway activated in Alzheimer’s disease. The Journal of cell biology 171 87 98
Zerbinatti C. V. Wahrle S. E. Kim H. Cam J. A. Bales K. Paul S. M. Holtzman D. M. Bu G. 2006Apolipoprotein E and low density lipoprotein receptor-related protein facilitate intraneuronal Abeta42 accumulation in amyloid model mice. The Journal of biological chemistry 281 36180 36186
Zhao X. L. Wang W. A. Tan J. X. Huang J. K. Zhang X. Zhang B. Z. Wang Y. H. Yang Cheng. H. Y. Zhu H. L. Sun X. J. et al. 2010Expression of beta-amyloid induced age-dependent presynaptic and axonal changes in