B R A I N R E S E A R C H R E V I E W S 6 7 ( 2 0 1 1 ) 1 8 4 – 1 9 2

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Review

Impaired regulation of synaptic actin cytoskeleton inAlzheimer's disease

Peter Penzesa,b,⁎, Jon-Eric VanLeeuwena

aDepartment of Physiology, Northwestern University Feinberg School of Medicine, 303 E. Chicago Avenue, Ward 7-174, Chicago, IL 60611, USAbDepartment of Psychiatry and Behavioral Sciences, Northwestern University Feinberg School of Medicine, 303 E. Chicago Avenue, Ward 7-174,Chicago, IL 60611, USA

A R T I C L E I N F O

⁎ Corresponding author at: Department of Phy7-174, Chicago, IL 60611, USA. Fax: +1 312 503

E-mail address: p-penzes@northwestern.e

0165-0173/$ – see front matter © 2011 Elsevidoi:10.1016/j.brainresrev.2011.01.003

A B S T R A C T

Article history:Accepted 19 January 2011Available online 26 January 2011

Representing themost commoncause of dementia, Alzheimer's disease (AD) has dramaticallyimpacted the neurological and economic health of our society. AD is a debilitatingneurodegenerative disease that produces marked cognitive decline. Much evidence hasaccumulated over the past decade to suggest soluble oligomers of beta-amyloid (Aβ) have acritical role in mediating AD pathology early in the disease process by perturbing synapticefficacy. Herewe critically review recent research that implicates synapses as key sites of earlypathogenesis in AD. Most excitatory synapses in the brain rely on dendritic spines as the sitesfor excitatory neurotransmission. The structure and function of dendritic spines aredynamically regulated by cellular pathways acting on the actin cytoskeleton. Numerousstudies analyzing human postmortem tissue, animal models and cellular paradigms indicatethat AD pathology has a deleterious effect on the pathways governing actin cytoskeletonstability. Based on the available evidence, we propose the idea that a contributing factor tosynaptic pathology in early AD is anAβ oligomer-initiated collapse of a “synaptic safety net” inspines, leading to dendritic spine degeneration and synaptic dysfunction. Spine stabilizingpathways may thus represent efficacious therapeutic targets for combating AD pathology.

© 2011 Elsevier B.V. All rights reserved.

Keywords:SynapseDendritic spineGlutamatergicPostmortemGeneticNeurodegenerativeAnimal modelCircuit

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1852. Dendritic spines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 185

2.1. Regulation of dendritic spines by actin dynamics. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1852.2. A role for spine dysregulation in AD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 185

3. Synaptic pathology in AD patients . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1854. AD susceptibility genes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 187

4.1. FAD genes: an indirect attack on synapses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1874.2. AD genetic risk factors alter dendrite and spine expression. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 187

5. Dendrite and synaptic pathology in AD mouse models . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 187

siology, Northwestern University Feinberg School of Medicine, 303 E. Chicago Avenue, Ward5101.du (P. Penzes).

er B.V. All rights reserved.

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6. Molecular mechanisms of AD pathogenesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1886.1. AD pathology potentiates synaptic-weakening pathways. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1896.2. AD pathology disrupts pathways regulating synaptic maintenance . . . . . . . . . . . . . . . . . . . . . . . 1896.3. Dysregulation of excitatory transmission in AD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 189

7. Conclusions: impaired “synaptic safety net” in spines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1898. Unanswered questions and future directions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 190Acknowledgments. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 190References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 190

1. Introduction

Alzheimer's disease (AD) is the most common cause ofdementia, affecting approximately 13% of Americans overthe age of 65 (Alzheimers, 2009). The widespread prevalenceof AD has taken a profound toll on our society, physically andeconomically. Globally, the total estimated costs of dementiaaccount for about 1% of the world's gross domestic product(www.alz.co.uk/research/worldreport 2009). Despite the tre-mendous impact of AD, deciphering its cause or developingeffective therapeutics for AD remains elusive. Patientssuffering from AD experience progressive loss of memory,critical reasoning and other cognitive abilities. Althoughamyloid deposits, neurofibrillary tangles (NFTs) and celldeath remain the defining characteristics of AD, the causalrelationship between these lesions and their contribution tocognitive impairment is still unclear. However, diverse lines ofevidence suggest beta-amyloid (Aβ) oligomers play a promi-nent role in AD pathology by inducing synapse degeneration.Here we critically review recent research that implicatessynapses as key sites of early pathogenesis in AD. Based onthese findings, we propose the idea that synaptic pathology inearly stages of AD is, at least in part, caused by the Aβoligomer-initiated collapse of a “synaptic safety net” inspines, which then leads to dysgenesis of dendritic spinesand loss of functional synapses. This synaptic pathology has alikely consequence of dendrite atrophy and, when combinedwith tau pathologies, cell death. Therapeutically enhancingspine stabilizing pathways may thus prevent or delay diseaseprogression.

2. Dendritic spines

The vast majority of excitatory synapses within the brainoccur on small dendritic protrusions, called dendritic spines(Fig. 1). Synaptic strength and neuronal function are greatlyinfluenced by dendritic spine size and number. Activity-dependent spine stability or remodeling contributes to themaintenance or rewiring of neuronal circuits during thelifespan (Alvarez & Sabatini, 2007; Zuo et al., 2005). Whilespines are highly dynamic early on, stable spines predominatethroughout adult life (Grutzendler et al., 2002). Despite thepredominance of stable spines in adulthood, imaging studiesdemonstrate experience-dependent structural alterations ofspines in live animals (Alvarez & Sabatini, 2007; Holtmaat &Svoboda, 2009), and spine expression has a salient associationwith human cognitive function (Ramakers, 2002).

2.1. Regulation of dendritic spines by actin dynamics

Spine stability is conferred by its actin cytoskeleton. Althougha relatively small fraction of actin in spines is stable (Kasai etal., 2003), actin stability is dynamically achieved throughcontinuous activity of actin-stabilizing signaling pathways. Ahost ofmolecules,most notably small GTPases suchasRac andRap, regulate actin dynamics. The activity of small GTPasesthemselves is under the control of GEFs, which serve asactivators of small GTPases. Depolimerization of actin leads tospine loss as well as loss of glutamate receptors from synapticsites (Allison et al., 1998; Halpain et al., 1998). Similarly,interference with the expression or function of upstreamregulators of the actin cytoskeleton, including Rac-GEFs, Rac,and Rac targets such as PAK cause spine and synapse loss(Tashiro et al., 2000; Cahill et al., 2009; Penzes et al., 2003; Xie etal., 2007). Thus the stable component of the actin cytoskeletonconfers the structural and functional integrity of the glutama-tergic synapse. Conversely, the dynamic fraction of the actincytoskeleton provides the driving force behind structuralremodeling of spines, and contributes to synaptic plasticity(Matus, 2005).

2.2. A role for spine dysregulation in AD

Interestingly, a number of proteins implicated in ADpathologyhave established roles in synaptic signaling. Furthermore,synapse degeneration is well supported as amajor componentof AD pathology (Selkoe, 2002). Based on the availableevidence, it is likely that spine dysgenesis, induced by Aβoligomers, contributes to disrupted neural networks anddecline of cognitive function in AD. Thus, continued researchinto spine destabilization associated with AD is important forunderstanding the disease process and developing effectivetherapeutics. Here we discuss the role of dendrite anddendritic spine perturbations in AD by reviewing neuropath-ological findings from AD patients, genetic factors that confersusceptibility to AD, evidence from transgenic mouse models,and potential molecular mechanisms underlying the diseaseprocess.

3. Synaptic pathology in AD patients

Synapse loss is a prominent and consistent finding inpostmortem tissue samples from patients diagnosed withAD (DeKosky & Scheff, 1990; Scheff et al., 1990). Quantitativemorphometric analysis of cortical biopsies within 2 to 4 years

Fig. 1 – Depiction of dendrites and dendritic spines found on cortical neurons. (A) Representative cultured cortical neuronexpressing green fluorescent protein (GFP). Dendrites extend from the soma and exhibit extensive branching, composing theneuron's dendritic arbor. Dendritic spines protrude from dendrites allowing neurons tomake synaptic connections. Notice thatthe axon is thinner than dendrites and does not have spines. (B) Magnified image of a dendrite expressing GFP. Dendritic spineshave a bulbous head, which serves as the site of synaptic contact, and a thinner neck connected to the dendrite. (C) Model of adendritic spine synapsing with an axon. The postsynaptic density (PSD) anchors synaptic proteins, including glutamatereceptors (e.g. NMDARs and AMPARs), at the synapse. The dynamic actin cytoskeleton confers much of the structure andfunction of the dendritic spine.

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of clinical AD onset demonstrated reduced numerical densityof synapses, ranging from 25% to 36%, and decreased synapsenumber per neuron (Davies et al., 1987). Numerous studiesreport dendritic spine loss in the hippocampus (DeKosky &Scheff, 1990; Ferrer & Gullotta, 1990) and throughout the cortex(Catala et al., 1988; Scheff & Price, 1993), the primary brainareas affected by AD-related pathology. Additionally, amyloidplaques found in the the brains of AD patients have beenassociated with dystrophic neurites (Ferrer et al., 1990). It haslong been suggested that aberrant structural changes in thecortex and hippocampus could cause cognitive impairment(Scheibel, 1979). Interestingly, cognitive decline has a strongercorrelation to synapse and dendrite loss than to NFTs orneuronal loss (DeKosky & Scheff, 1990; Terry et al., 1991).Stereologic sampling of autopsy tissue demonstrated synapseloss in the hippocampus, consisting in a (non-significant) 13%reduction of synapse numbers in mild cognitive impairment(MCI) and a 44% loss in early AD patients (Scheff et al., 2007),indicating that synapse loss represents an early insult in AD

that advances with the disease. Furthermore, it has beenreported that synapse loss often appears greater than whatwould be expected from the neuronal death that also occurswith AD, suggesting synapse loss has a central role in ADpathogenesis, rather than just a consequence of cell death(Walsh & Selkoe, 2004). Interestingly, non-demented indivi-duals with a genetic predisposition for developing AD later inlife display decreased cortical thickness when compared tocontrol subjects (Apostolova & Thompson, 2008). Findings thatindicate synapse deterioration begins early in AD underscorethe need to develop better diagnostic tools and enhance ourunderstanding of the neurological changes that take placeduring the early stages of AD. Approaches to stall or reversedisease progression will likely be most efficacious during thisperiod. That neuropathological studies note putative compen-satory changes in the AD brain, such as an increase in the sizeof remaining dendritic spines, indicates inherent neurologicalprocesses capable of combating AD pathology may exist (Fialaet al., 2002; Scheff & Price, 1993). Such mechanisms should be

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further investigated, as they could represent ideal modifiablepathways suitable for therapeutic targeting to delay diseaseprogression.

4. AD susceptibility genes

4.1. FAD genes: an indirect attack on synapses

Genetic findings have served as an important guide to ADresearch, but the role specific genes play in contributing to ADpathology remains unclear and convoluted. However, recentfindings have identified new candidate susceptibility genesand new roles for established AD associated genes that shouldmerit the focus of future studies. Familial AD (FAD), which hasan autosomal dominant form of inheritance, has beenassociated with mutations in three genes that are involvedwith the production of Aβ (Bertram & Tanzi, 2008). Amyloidprecursorprotein (APP),whencleaved sequentially byβ- andγ-secretase, produces the Aβ peptide. All FAD-associated muta-tions are found in APP or genes that make up the γ-secretasecomplex: presenilin 1 (PSEN1) and presenilin 2 (PSEN2).Mutations associated with FAD lead to increased productionof Aβ, and much evidence from cellular studies convincinglydemonstrate that soluble Aβ oligomers impair synapticplasticity (reviewed in(Klein, 2006; Selkoe, 2008)). Studiesshow that Aβ oligomers target dendritic spines therebyinducing spine dysgenesis and reductions in spine density(Lacor et al., 2007; Shankar et al., 2007).

4.2. AD genetic risk factors alter dendrite andspine expression

The vast majority of AD cases have a late-onset development,sometimes referred to as sporadic AD. Although hundreds ofgenes have been proposed as AD risk factors, the geneencoding apolipoprotein E (APOE) has become established asthe most important risk factor (Corder et al., 1993; Sleegers etal., 2010). Carriers of the ε4 (APOE ε4) allele are at higher risk fordeveloping AD, whereas evidence suggests the ε2 allele isneuroprotective. Interestingly, recent studies using transgenicmice indicate that ApoE isoforms differentially influencedendrite anddendritic spinemorphology. Specifically, reducedspine density was observed in the dentate gyrus (DG) of miceexpressing human APOE ε4 when compared mice expressinghuman APOE ε3 or WT mice (Ji et al., 2003). The authors alsoinvestigated APOE ε4 expression in human patients and foundan inverse correlation betweenAPOE ε4 dose and spine densityin the DG. In another study, expression of human APOE ε4 inmice lead to reduced dendritic length and branching through-out the cortex and hippocampus (Dumanis et al., 2009). Thisstudy did not find that ApoE isoforms affect spine expressionin the hippocampus, but the authors did report differences inthe cortex. It has also been reported that hippocampal spineloss normally observed in an AD mouse model can beprevented by overexpressing human ApoE2, maintainingspine density at control levels (Lanz et al., 2003). Studiescontinue to reaffirm the importance of APOE as an AD geneticrisk factor and although its precise role in AD pathologyremains unclear, it is fascinating that differential APOE

expression can lead to changes in dendrites and dendriticspines, indicating that genetic mutations that lead to thedevelopment of AD may impact neuronal structural stability.

Two independent genome-wide association studies(GWAS) recently identified the clusterin gene (CLU) as a newsusceptibility gene for AD (Sleegers et al., 2010). Clusterin, alsoknown as ApoJ, has many similarities to ApoE, including theability to bind Aβ, thus it will be interesting to learn if clusterinalso modulates expression of dendrites and dendritic spines.Interestingly, PICALM, another novel susceptibility geneidentified by GWAS may influence dendrite structure. With aknown role in clathrin-mediated endocytosis, PICALM canalso induce dendritic dystrophy and disrupt vesicle transportwhen underexpressed in embryonic hippocampal neurons(Sleegers et al., 2010). As new genetic risk factors are identifiedand manipulated experimentally, it will be important toassess dendrite and dendritic spine phenotypes, and whethergenotype alters neuronal structure through a gain-of-functionor loss-of-function phenotype. If expression of a specificgenetic mutation displays a reduced spine density phenotype,is it due to overproduction of Aβ or inherently compromisingsynaptic stability?

5. Dendrite and synaptic pathology in ADmouse models

Based on genetic links identified from the human populationassociated with developing AD, several transgenic mousemodels have been generated to recapitulate specific aspects ofAD pathology. Interestingly, rare mutations appear capable ofproducing the full scope of AD pathology in humans, but whenexpressed in mice, these transgenes recapitulate only specificaspects of AD (Ashe & Zahs, 2010). Despite their shortcomings,AD animal models have been extraordinarily instructive.Given that aberrant APP processing is suggested to temporallyprecede tau alterations and has been directly linked withspine degeneration (Selkoe, 2002), this review will emphasizeanimal models that mimic amyloidogenesis.

Due to the profound memory loss associated with AD, it isnecessary to test AD mouse models for behavioral deficitsindicative of cognitive impairment, especially impairment inreferencememoryandworkingmemory.Manymodels displaymemory impairment as well as aberrant LTP expression (Ashe& Zahs, 2010). Additionally, the prominent synapse pathologyobserved in AD patients has prompted the investigation ofspine morphology and density in ADmodels. The widely usedTg2576 mouse model (Hsiao et al., 1996), which expresseshumanAPP containingmutations identified in a large Swedishfamily, display decreased spine density in CA1 and DG muchbefore the formation of amyloid plaques, supporting a role forAβ oligomers inmediating at least some AD-related pathology(Jacobsen et al., 2006; Lanz et al., 2003). Interestingly, cognitiveimpairment also becomes evident in thesemiceprior to plaquedevelopment but around the time when spines becomedepleted, suggesting that synapse loss can drive cognitivedecline. Expressing mutations in APP and PS1 in mice leads toneurons with fewer large spines and various dendriticabnormalities (Knafo et al., 2009). Such dendritic abnormalitiesinclude shaft atrophy, neurite breakage, and greater

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reductions in spinedensitynear amyloiddeposits (Grutzendleret al., 2007; Tsai et al., 2004). Similarly, amyloid plaques inTg2576 mice alter neurites and reduce spine density ondendrites nearby (Spires et al., 2005). Taken together, thesestudies suggest that both soluble and insoluble amyloid canhave deleterious effects on neurons by perturbing synapticconnections as well as dendritic projections. It should benoted, though, that substantial synapse degeneration appearsto take place prior to plaque deposition. It will thus beimportant to explore dendrite and spine phenotypes in newlygenerated animal models and at time-points before wide-spread deposition of amyloid plaques. Many AD animalmodels support the concept that synaptic degeneration iscentral to the disease and may serve as a driving force, ratherthan a byproduct, of AD pathology that leads to memoryimpairment. Importantly, structural alterations have beenreported to be reversible pharmacologically, opening newtherapeutic directions in AD (Smith et al., 2009).

Fig. 2 – AD-related pathology causes synaptic dysfunction. (A) Custained for drebrin (blue) and treated with fluorescent Aβ oligomdemonstrating ADDLs bind excitatory synapses. (B) Dendrite andADDLs (cyan). Aβ oligomers clearly bind dendritic spines, with cSoluble Aβ oligomers downregulatemany of the keymolecular palso upregulate the activity of proteins that weaken the actin cytmechanisms regulating synaptic plasticity become dysregulatedcollapse of the dendritic spine and ultimately loss of functional s

Despitemajor advancesmade possible by the use of animalmodels, the available models are incomplete and the findingsthey produce should be taken in conjunction with thelimitations of each model. Most models mimic only one or afew components of AD, thus they provide insight about anarrow aspect of the disease, not necessarily the disease as awhole. While genetic manipulations in animals may helpidentify the role of individual proteins in AD pathogenesis,they could also elucidate important common pathwaysaffected in the disease. Determining the specific pathwaysdisrupted in AD and understanding how they contribute to ADpathology is thus an important next step.

6. Molecular mechanisms of AD pathogenesis

A wealth of genetic data provided evidence that Aβ has a keyrole in AD pathogenesis, which has been corroborated by

ltured cortical neuron over-expressing kalirin-7 (green),ers (also called ADDLs, red). White indicates co-localization,spines of a cortical neuron (green) treated with fluorescent

o-localization shown in white (indicated by grey arrows). (C)layers responsible formaintaining the actin cytoskeleton, andoskeleton (loss of the “synaptic safety net”). As a result, thewhereby increased actin destabilization could precipitateynapses.

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numerous molecular studies and furthered by results showingthat Aβ can mediate its toxic effects by acting on synapses (Fig.2). Given the dramatic synaptic alterations in AD, it is notsurprising that AD patients demonstrate altered expression ofmany synaptic proteins (Arendt, 2009). The presynaptic proteinsynaptophysinwas reported to be reducedby 25-35% inpatientsdiagnosed with MCI or AD (Masliah et al., 2001; Selkoe, 2002).However, since synaptophysin knockout mice lack deficits insynaptic plasticity and cognition, it has been suggested thatmemory decline in AD involves much more than just synapto-physin loss (Janz et al., 1999; Zhao et al., 2006). Investigation intopostsynaptic signaling molecules has revealed a number ofintriguing proteins of interest. Although the precise mecha-nisms that cause spine degeneration in AD remain unknown,recent findings suggest that signalingpathways regulating actindynamics and receptor expression may be integrally involved.

6.1. AD pathology potentiates synaptic-weakeningpathways

The actin binding proteins cofilin and drebrin have contrast-ing effects on actin stability and their regulated activity worksin conjunction to control actin dynamics. Active cofilininduces actin destabilization, and much evidence supports arole for cofilin in neurodegeneration, including AD (Maloney &Bamburg, 2007; Shankar et al., 2007). In contrast, drebrin bindsand stabilizes actin in dendritic spines, but reduced levels ofdrebrin have been reported in the hippocampal formations ofpatients with AD (Harigaya et al., 1996) and in cortical areas,including the frontal and temporal cortices (Counts et al.,2006). Cofilin and drebrin are direct regulators of actin that aredisrupted in AD, but molecules further upstream in actinregulatory pathways are impacted as well.

Just as there are signaling pathways that enhance synapticstrength and regulate synaptic maintenance, other pathwaysmediate synaptic weakening. Calcineurin (CaN or PP2B) is acalcium sensitive phosphatase that, when activated, caninduce a signaling cascade leading to synaptic weakening(Xia & Storm, 2005). Interestingly, calcineurin over-activationhas been reported in AD patients and animal models (Liu et al.,2005). Studies have also shown that Aβ oligomer-inducedspine loss and dendritic dystrophies can be prevented bycalcineurin inhibition (Wu et al., 2010). Furthermore, adownstream effector molecule of calcineurin, GSK-3β, experi-ences increased activation in response to AD-related pathology(Li et al., 2009). Hence over-activation of an NMDAR-calci-neurin-GSK-3β pathwaymay represent a mechanism by whichsynapses degenerate in AD (Fig. 2C).

6.2. AD pathology disrupts pathways regulating synapticmaintenance

A critical regulator of actin assembly and subsequent spinemodulation in neurons is p21-activated kinases (PAK), whichsignals downstream of Rac (Penzes et al., 2003; Zhao et al.,2006). In the hippocampus of AD patients, and animal modelsof AD, total PAK is reduced with an even greater reduction inactive PAK (Zhao et al., 2006). AD-related pathology alsomislocalizes PAK in neurons, followed by loss of F-actin indendrites and dendritic spines (Ma et al., 2008). Furthermore,

pharmacological inhibition of PAK in adultmicewas sufficientto cause memory impairment, cofilin pathology and drebrinloss (Zhao et al., 2006). Interestingly, kalirin, a key regulator ofspine morphogenesis and an upstream activator of PAK inspines, was found to have consistently underexpressedprotein and mRNA levels in the hippocampus of AD patients,suggesting a role for the kalirin-Rac-PAK pathway in AD-associated spine pathology (Xie et al., 2007; Youn et al., 2007)(Fig. 2C). Studies investigating protein expression in ADpatients continue to reveal aberrant expression of signalingmolecules that regulate spine dynamics. Another smallRhoGTPase that influences synaptic plasticity by regulatingcytoskeleton dynamics called RhoA was also recently de-scribed to have reduced expression and altered localization inAD brain hippocampus (Huesa et al., 2010).

6.3. Dysregulation of excitatory transmission in AD

Just as the pathways governing spine plasticity are dysregu-lated in AD, so too are many receptors that mediate synaptictransmission, causing impairments in functional activity (Fig.2). As dendritic spines serve as sites for most excitatorycommunication in the brain, numerous studies have investi-gated the effect of AD pathology on the receptors that mediateglutamatergic transmission. Interestingly, Aβ reduces surfaceexpression of NMDARs and AMPARs, resulting in dendriticspine loss (Hsieh et al., 2006; Snyder et al., 2005). Notsurprisingly, various studies have shown that Aβ oligomersinhibit LTP (Lambert et al., 1998; Walsh et al., 2002). Animportant modulator of NMDAR activity, Fyn, a tyrosinekinase, has been reported to be upregulated in AD (Chin etal., 2005). Moreover, overexpressing Fyn concomitantly withAβ exposure produces neuronal and cognitive dysfunction inmice, whereas Fyn depletion prevents the neurotoxic effectsof Aβ (Lambert et al., 1998; Venkitaramani et al., 2007).Interestingly, a recent study demonstrated postsynaptictargeting of Fyn is mediated by tau and that sequestration ofFyn to the soma was sufficient to ameliorate pathology in anAD mouse model (Ittner et al., 2010). AD pathology alsoappears to affect metabotropic glutamate receptors (mGluRs).Recently, Aβ oligomers were shown to cluster mGluR5 therebyinhibiting its synaptic diffusion, which led to increasedintracellular calcium and synaptic degeneration (Renner etal., 2010). As determining the molecular processes underlyingsynapse degeneration in AD is critical for understanding thedisease and for developing effective therapeutics, the precisesignaling cascades underlying synapse loss and cognitivedecline need to be further elucidated.

7. Conclusions: impaired “synaptic safety net”in spines

Synapse degeneration is nowwell established as amajor earlystep in AD pathology. Synapse loss tightly correlates withcognitive decline, and AD-associated etiologic factors havebeen shown to adversely affect dendritic spines in vitro and invivo. The fact that AD is a disease of aging, could suggestimpairment in spine maintenance. Postmortem neuropatho-logical findings, along with animal models and cellular

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studies, indicate that the pathways regulating the stability andremodeling of the actin cytoskeleton are impaired in AD.Disruption of actin regulatory pathways appears to occur atvarying levels of the signaling cascade, including directregulators of actin as well as molecules further upstream,such as GEFs and small GTPases. As the actin cytoskeletonprovides the structural scaffold for spines and synapses, andanchors neurotransmitter receptors at the synapse, degener-ation of signaling that maintains spines will ultimately lead tocollapse of spines and loss of synapses (Allison et al., 1998;Halpain et al., 1998). A stable synaptic cytoskeleton may serveas a “synaptic safety net” that could protect spiny synapsesfrom elimination, and prevent the loss of their glutamatereceptor content. Thus maintaining the functional integrity ofthis “synaptic safety net” could be crucial in preventing ordelaying synapse loss. We propose that by therapeuticallyenhancing actin stability one may be able to delay synapseloss when targeted early in disease progression, even inprodromal stages. While cell surface receptors are the mosteasily targeted proteins, several components of the intracel-lular pathway that regulates the actin cytoskeleton, such asGEFs or kinases, might have favorable druggability properties.

8. Unanswered questions and future directions

Although much progress has been made regarding spinedysmorphogenesis in AD, many questions still need to beaddressed. How do Aβ oligomers exert their effect onsynapses? While studies demonstrate Aβ oligomers do binddendritic spines, their mode of action on the cytoskeleton isunknown. Future studies should address the missing linksbetween Aβ oligomer binding to spines and cytoskeletalalterations. It will be valuable to know whether Aβ binding ismediated by high affinity interactions with specific regions ofthe cell membrane or if a specific receptor exists thatfacilitates Aβ toxicity.

What are the key synaptic signaling pathways disrupted inAD and how are they connected to other AD-related pathology,such as tau pathology and cell death? A number of signalingmolecules that regulate synapse function have been shown tobe dysregulated in AD, but it will be important to differentiateaffected proteins from those that mediate pathology. Further-more, the mechanisms induced by Aβ-oligomers that lead tosynapse degeneration are becoming known, with a strengthe-ning causal link to tau pathology, including a potential role indendrite degeneration (Ittner et al., 2010; Zempel et al., 2010).Progress has been made toward reconciling the relationshipbetween synaptic pathology and neuronal loss but it remains alingering issue that needs resolution.

What role do genetic risk factors play in AD-related spinedestabilization? The ε4 allele of APOE, which stands as thestrongest genetic risk factor for developing AD, has beenshown to alter dendrite and dendritic spine expression(Dumanis et al., 2009; Ji et al., 2003). However, the mechanismby which ApoE exerts its dendritic effects remains unknownand warrants further investigation. Moreover, recent geneticstudies provide compelling evidence for newly identified ADsusceptibility genes, which represent exciting new opportu-nities to decipher the molecular basis of AD pathology.

Can synapse degeneration be prevented or reversed?Addressing the questions above will hopefully lead towindows of opportunity for intervention. Discovering thekey players in spine destabilization will engender approachesto synapse protection and restoration. Synapse loss likelyprecedes cell loss, thus, discovering means to slow or haltsynapse degeneration may also preserve cell health. Manystudies indicate that synapse degeneration contributes tocognitive decline and so mitigating spine loss may alsoameliorate cognitive impairment.

Acknowledgments

This work was supported by grants from NIH-NIMH(MH071316, MH071533), National Alliance for Research onSchizophrenia and Depression (NARSAD), and Alzheimer'sAssociation (to P.P.), and NIH 1F31MH087043 (J.V.).

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