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Alzheimer Disease Developments – Spring 2015

Larry H. Bernstein, MD, FCAP, Curator

LPBI

 

 

Cognitive Stimulation Modulates Platelet Total Phospholipases A2 Activity in Subjects with Mild Cognitive Impairment

 

JNK: A Putative Link Between Insulin Signaling and VGLUT1 in Alzheimer’s Disease

Omega-3 Fatty Acid Status Enhances the Prevention of Cognitive Decline by B Vitamins in Mild Cognitive ImpairmentOpenly Available
Oulhaj, Abderrahim | Jernerén, Fredrik | Refsum, Helga | Smith, A. David | de Jager, Celeste A.

Preliminary Study of Plasma Exosomal Tau as a Potential Biomarker for Chronic Traumatic EncephalopathyOpenly Available
Stern, Robert A. | Tripodis, Yorghos | Baugh, Christine M. | Fritts, Nathan G. | Martin, Brett M. | Chaisson, Christine | Cantu, Robert C. | Joyce, James A. | Shah, Sahil | Ikezu, Tsuneya | Zhang, Jing | Gercel-Taylor, Cicek | Taylor, Douglas D

AZD3293: A Novel, Orally Active BACE1 Inhibitor with High Potency and Permeability and Markedly Slow Off-Rate KineticsOpenly Available
Eketjäll, Susanna | Janson, Juliette | Kaspersson, Karin | Bogstedt, Anna | Jeppsson, Fredrik | Fälting, Johanna | Haeberlein, Samantha Budd | Kugler, Alan R. | Alexander, Robert C. | Cebers, Gvido

Predictive Value of Cerebrospinal Fluid Visinin-Like Protein-1 Levels for Alzheimer’s Disease Early Detection and Differential Diagnosis in Patients with Mild Cognitive Impairment
Babić Leko, Mirjana | Borovečki, Fran | Dejanović, Nenad | Hof, Patrick R. | Šimić, Goran

Plasma Phospholipid and Sphingolipid Alterations in Presenilin1 Mutation Carriers: A Pilot Study
Chatterjee, Pratishtha | Lim, Wei L.F. | Shui, Guanghou | Gupta, Veer B. | James, Ian | …… | Wenk, Marcus R. | Bateman, Randall J. | Morris, John C. | Martins, Ralph N.

Cognitive reserve in ageing and Alzheimer’s disease / Stern Y / Lancet Neurol. 2012 Nov; 11(11):1006-12. PMID: 23079557.

A mutation in APP protects against Alzheimer’s disease and age-related cognitive decline/ Jonsson T, Atwal JK, Steinberg S, Snaedal J, Jonsson PV, Bjornsson S, Stefansson H, Sulem P, Gudbjartsson D, Maloney J, et al. / Nature. 2012 Aug 2; 488(7409):96-9. PMID: 22801501.

 Propagation of tau pathology in a model of early Alzheimer’s disease / de Calignon A, Polydoro M, Suárez-Calvet M, William C, Adamowicz DH, Kopeikina KJ, Pitstick R, Sahara N, Ashe KH, Carlson GA, et al. / Neuron. 2012 Feb 23; 73(4):685-97. PMID: 22365544.

Stages of the pathologic process in Alzheimer disease: age categories from 1 to 100 years/ Braak H, Thal DR, Ghebremedhin E, Del Tredici K / J Neuropathol Exp Neurol. 2011 Nov; 70(11):960-9. PMID: 22002422.

Neuroinflammation in Alzheimer’s disease and mild cognitive impairment: a field in its infancy / McGeer EG, McGeer PL / J Alzheimers Dis. 2010; 19(1):355-61. PMID: 20061650.

Metallothioneins in Prion- and Amyloid-Related Diseases

MICROGLIA

Microglia are the immune cells of the CNS and account for approximately 10% of the CNS cellpopulation, with regional variation in density [27, 28]. During embryonic development, microglia originate from yolk sac progenitor cells that migrate into the developing CNS during early embryogenesis [29,30].Following construction of the blood-brain barrier (BBB), microglia are renewed by local turnover [31]. In the healthy brain, microglia actively support neurons through the release of insulin-like growth factor 1, nerve growth factor, ciliary neurotrophic factor, and brain-derived neurotrophic factor (BDNF) [32–34]. Microglia also provide indirect support to neurons by clearance of debris to maintain the extracellular environment, and phagocytosis of apoptotic cells to facilitate neurogenesis [35, 36]. In the adult brain, microglia coordinate much of their activity with astrocytes and activate in response to similar stimuli [37, 38]. Dysfunctional signaling between microglia and astrocytes often results in chronic inflammation, a characteristic of many neurodegenerative diseases [39, 40].

Historically, it has been thought that microglia ‘rest’ when not responding to inflammatory stimuli or damage [41, 42]. However, this notion is being increasingly recognized as inaccurate [43]. When not involved in active inflammatory signaling, microglia constantly patrol the neuropil by extension and retraction of their finely branched processes [44]. Microglial activation is often broadly classified into two states; pro-inflammatory (M1) or anti-inflammatory (M2) [36, 45], based on similar phenotypes in peripheral macrophages [46]. M1 activated microglia are characterized by increased expression of pro-inflammatory mediators and cytokines, including inducible nitric oxide synthase, tumor necrosis factor-α, and interleukin-1β, often under the control of the transcription factor nuclear factor-κB [45]. Pro-inflammatory microglia rapidly retract their processes and adopt an amoeboid morphology and often migrate closer to the site of injury [47]. Anti-inflammatory M2 activation of microglia, often referred to as alternative activation, represents the other side of microglial behavior. Anti-inflammatory activation is characterized by increased expression of cytokines including arginase 1 and interleukin-10, and is associated with increased ramification of processes [45]. The polarization of microglia into M1 or M2 throughout the brain is well characterized, especially in neurodegenerative diseases [48]. In the AD brain, microglia expressing markers of M1 activation are typically localized to brain regions such as the hippocampus that are most heavily affected in the disease [49]. However, it is important to note that M1 and M2 classifications of microglia may over-simplify microglial phenotypes and may only represent the extremes of microglial activation [50]. It has been more recently proposed that microglia likely occupy a continuum between these phenotypes [39, 51].

Do microglia have multiple roles in AD?

Classical pro-inflammatory activation of microglia has long been associated with AD [39, 49]. Samples taken from late-stage AD brains contain characteristic signs of inflammation, including amoeboid morphology of microglia, high levels of pro-inflammatory cytokines in the cerebrospinal fluid, and evidence of neuronal damage due to chronic exposure to pro-inflammatory cytokines and oxidative stress [52, 53]. The cause of this inflammation may be in response to direct toxicity of Aβ to neurons resulting in activation of nearby microglia and astrocytes [53, 54]. However, Aβ may also induce inflammatory activation of microglia and astrocytes. Activated immune cells are typically present surrounding amyloid plaques [55–57], with such peri-plaque cells exhibiting strong evidence of pro-inflammatory activation [56, 58–60]. The presence of undigested Aβ particles within these activated microglia may suggest that the Aβ peptide itself is a pro-inflammatory signal for microglia [61–64]. In vitro experiments provide supporting evidence for the in vivo studies, with Aβ promoting pro-inflammatory microglial activation [65, 66], and also acting as a potent chemotactic signal [67].

However, it is important to note that although widespread inflammation is characteristic of late-stage AD, it remains unclear what role inflammation could play in early stages of the disease. Some evidence suggests that reducing inflammation through the long-term use of some non-steroidal anti-inflammatory drugs (NSAIDs) can reduce the risk of AD [68]. However, these findings have not yet been verified in clinical trials [69, 70]. Little is understood about how NSAIDs and related compounds affect the delicate balance of pro- versus anti-inflammatory microglial activity within the brain. Although there is considerable evidence to suggest that chronic inflammation may contribute to pathology in the later stages of AD, it is important to note that inflammation normally only represents a small aspect of microglial function. The non-inflammatory functions of microglia may play a more important role in early disease; specifically, microglial functions relating to maintenance of the CNS.

Phagocytosis: A vital role of microglia that may be lost in AD    

SYNAPTIC PRUNING: MICROGLIA CAN REGULATE NETWORK ACTIVITY

Recently, a new function has been proposed for microglia. A number of studies have provided evidence that microglia prune synapses throughout life. Microglia are known to remove extraneous synapses during development to ensure that only meaningful connections remain [43]. It was, however, thought that differentiated astrocytes performed the majority of synaptic pruning in the adult brain [91]. The discovery that microglial processes are constantly active within the brain and are often positioned near synapses raised the question of whether microglial synaptic pruning continued throughout life [44, 47, 92–94]. This question was answered in 2014 in a study that demonstrated that microglia do prune synapses into adulthood, and that this activity is important for normal brain function [95]. These findings supported those found a year earlier in a study reporting that ablation of microglia from brain slices increases synapse density and results in abnormal firing of hippocampalneurons [96].

Altered microglial behavior may underlie altered neuronal firing in AD  

Altered neuronal activity is an early phenomenon in AD

The cause of DMN hypoactivity in AD is not yet clear; however studies performed in cohorts that are genetically predisposed to AD suggest that DMN hypoactivity is preceded by a period of hyperactivity and increased functional connectivity [123, 136], often manifesting as an absence of normal DMN deactivation during external tasks [137–140]. DMN hyperactivity may interfere with hippocampal memory encoding, leading to the memory deficits that are present in mild cognitive impairment [141, 142]. It has been proposed that hippocampal hyperexcitability in AD may develop as a protective mechanism against increased input from the DMN [142–144]. As AD progresses, the initial hyperexcitability of the DMN and hippocampus may result in hypoactivity due to exhaustion of compensatory mechanisms [123, 136]. Evidence from both transgenic AD mice and longitudinal human studies supports an exhaustion model of hyperactivation leading to later hypoactivation [143, 145–147]. Interestingly, a number of studies report a lower incidence of AD among those who regularly practice meditation which specifically ‘calms’ the DMN [148].

Our understanding of AD as a disease is changing. Historically considered to be primarily a disease of neuronal degeneration, this neurocentric view has widened to encompass non-neuronal cells such as astrocytes into our understanding of the disease process and pathogenesis. A proposed model for microglia in AD is shown in Fig. 2. Microglia perform a wide range of functions in the CNS and although this includes induction of an inflammatory reaction in response to damage, they also have critical roles for maintaining normal function in the brain. Recent evidence shows that microglia regulate neuronal activity through synaptic pruning throughout life as an extension on their normal phagocytosis behavior. The discovery of a large number of AD risk genes associated with reduced immune cell function suggests that perturbed microglial phagocytosis could lead to AD. In our model, altered microglial phagocytosis of synapses results in network dysfunction and onset of AD, occurring downstream of Aβ.

The immune system and microglia represent a novel target for intervention in AD. Importantly, a large number of anti-inflammatory drugs are already in use for other conditions. What is important to know at this stage is exactly how to best target immune cell function. The studies outlined here provide evidence that an indiscriminate dampening down of all microglial activity may result in a worse outcome for individuals by suppressing normal microglial regulatory functions. We currently do not know whether future microglial-based therapies should focus on reducing chronic inflammation or conversely, whether they should be aimed at boosting microglial phagocytosis. It is also likely that future treatment strategies may use a combination of approaches to target Aβ, immune cell phagocytosis and network activity. An increasing view in the AD field is that any drug or therapy needs to be provided very early in the disease process to maximize its beneficial effects. Although we are currently unable to effectively target those at risk of AD at such an early stage, advances in neuroimaging for subtle changes in network activity, or in assays for immune cell function, may provide new avenues for identification of early damage and risk of disease.

REFERENCES

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Selkoe DJ ((2011) ) Alzheimer’s disease. Cold Spring Harb Perspect Biol 3: , pii: a004457.

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Masters CL , Simms G , Weinman NA , Multhaup G , McDonald BL , Beyreuther K ((1985) ) Amyloid plaque core protein in Alzheimer disease and Down syndrome. Proc Natl Acad Sci U S A 82: , 4245–4249.

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Glenner GG , Wong CW ((1984) ) Alzheimer’s disease: Initial report of the purification and characterization of a novel cerebrovascular amyloid protein. Biochem Biophys Res Commun 120: , 885–890.

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Goldgaber D , Lerman MI , McBride OW , Saffiotti U , Gajdusek DC ((1987) ) Characterization and chromosomal localization of a cDNA encoding brain amyloid of Alzheimer’s disease. Science 235: , 877–880.

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Kang J , Lemaire HG , Unterbeck A , Salbaum JM , Masters CL , Grzeschik KH , Multhaup G , Beyreuther K , Muller-Hill B ((1987) ) The precursor of Alzheimer’s disease amyloid A4 protein resembles a cell-surface receptor. Nature 325: , 733–736.

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Robakis NK , Ramakrishna N , Wolfe G , Wisniewski HM ((1987) ) Molecular cloning and characterization of a cDNA encoding the cerebrovascular and the neuritic plaque amyloid peptides. Proc Natl Acad Sci U S A 84: , 4190–4194.

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Levy E , Carman MD , Fernandez-Madrid IJ , Power MD , Lieberburg I , van Duinen SG , Bots GT , Luyendijk W , Frangione B ((1990) ) Mutation of the Alzheimer’s disease amyloid gene in hereditary cerebral hemorrhage, Dutch type. Science 248: , 1124–1126.

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Levy-Lahad E , Wasco W , Poorkaj P , Romano DM , Oshima J , Pettingell WH , Yu CE , Jondro PD , Schmidt SD , Wang K , et al ((1995) ) Candidate gene for the chromosome 1 familial Alzheimer’s disease locus. Science 269: , 973–977.

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Rogaev EI , Sherrington R , Rogaeva EA , Levesque G , Ikeda M , Liang Y , Chi H , Lin C , Holman K , Tsuda T , et al ((1995) ) Familial Alzheimer’s disease in kindreds with missense mutations in a gene on chromosome 1 related to the Alzheimer’s disease type 3 gene. Nature 376: , 775–778.

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Sherrington R , Rogaev EI , Liang Y , Rogaeva EA , Levesque G , Ikeda M , Chi H , Lin C , Li G , Holman K , Tsuda T , Mar L , Foncin JF , Bruni AC , Montesi MP , Sorbi S , Rainero I , Pinessi L , Nee L , Chumakov I , Pollen D , Brookes A , Sanseau P , Polinsky RJ , Wasco W , Da Silva HA , Haines JL , Perkicak-Vance MA , Tanzi RE , Roses AD , Fraser PE , Rommens JM , St George-Hyslop PH ((1995) ) Cloning of a gene bearing missense mutations in early-onset familial Alzheimer’s disease. Nature 375: , 754–760.

 

Late-Onset Metachromatic Leukodystrophy with Early Onset Dementia Associated with a Novel Missense Mutation in the Arylsulfatase A Gene

Microbes and Alzheimer’s DiseaseOpenly Available
Itzhaki, Ruth F. | Lathe, Richard | Balin, Brian J. | Ball, Melvyn J. | Bearer, Elaine L. | Braak, Heiko | Bullido, Maria J. | Carter, Chris | Clerici, Mario | Cosby, S. Louise | Del Tredici, Kelly | Field, Hugh | Fulop, Tamas | Grassi, Claudio | Griffin, W. Sue T. | Haas, Jürgen | Hudson, Alan P. | Kamer, Angela R. | Kell, Douglas B. | Licastro, Federico | Letenneur, Luc | Lövheim, Hugo | Mancuso, Roberta | Miklossy, Judith | Otth, Carola | Palamara, Anna Teresa | Perry, George | Preston, Christopher | Pretorius, Etheresia | Strandberg, Timo | Tabet, Naji | Taylor-Robinson, Simon D. | Whittum-Hudson, Judith A.

Longitudinal Relationships between Caloric Expenditure and Gray Matter in the Cardiovascular Health StudyOpenly Available
Raji, Cyrus A. | Merrill, David A. | Eyre, Harris | Mallam, Sravya | Torosyan, Nare | Erickson, Kirk I. | Lopez, Oscar L. | Becker, James T. | Carmichael, Owen T. | Gach, H. Michael | Thompson, Paul M. | Longstreth Jr., W.T. | Kuller, Lewis H.

Preliminary Study of Plasma Exosomal Tau as a Potential Biomarker for Chronic Traumatic EncephalopathyOpenly Available
Stern, Robert A. | Tripodis, Yorghos | Baugh, Christine M. | Fritts, Nathan G. | Martin, Brett M. | Chaisson, Christine | Cantu, Robert C. | Joyce, James A. | Shah, Sahil | Ikezu, Tsuneya | Zhang, Jing | Gercel-Taylor, Cicek | Taylor, Douglas D.

Unraveling Alzheimer’s: Making Sense of the Relationship between Diabetes and Alzheimer’s Disease1Openly Available
Schilling, Melissa A.

Pain Assessment in Elderly with Behavioral and Psychological Symptoms of DementiaOpenly Available
Malara, Alba | De Biase, Giuseppe Andrea | Bettarini, Francesco | Ceravolo, Francesco | Di Cello, Serena | Garo, Michele | Praino, Francesco | Settembrini, Vincenzo | Sgrò, Giovanni | Spadea, Fausto | Rispoli, Vincenzo

Editor’s Choice from Volume 50, Number 4 / 2016

Post Hoc Analyses of ApoE Genotype-Defined Subgroups in Clinical Trials
Kennedy, Richard E. | Cutter, Gary R. | Wang, Guoqiao | Schneider, Lon S.

Protective Effect of Amyloid-β Peptides Against Herpes Simplex Virus-1 Infection in a Neuronal Cell Culture Model
Bourgade, Karine | Le Page, Aurélie | Bocti, Christian | Witkowski, Jacek M. | Dupuis, Gilles | Frost, Eric H. | Fülöp, Tamás

Association Between Serum Ceruloplasmin Specific Activity and Risk of Alzheimer’s Disease
Siotto, Mariacristina | Simonelli, Ilaria | Pasqualetti, Patrizio | Mariani, Stefania | Caprara, Deborah | Bucossi, Serena | Ventriglia, Mariacarla | Molinario, Rossana | Antenucci, Mirca | Rongioletti, Mauro | Rossini, Paolo Maria | Squitti, Rosanna

Effects of Hypertension and Anti-Hypertensive Treatment on Amyloid-β (Aβ) Plaque Load and Aβ-Synthesizing and Aβ-Degrading Enzymes in Frontal Cortex
Ashby, Emma L. | Miners, James S. | Kehoe , Patrick G. | Love, Seth

AZD3293: A Novel, Orally Active BACE1 Inhibitor with High Potency and Permeability and Markedly Slow Off-Rate KineticsOpenly Available
Eketjäll, Susanna | Janson, Juliette | Kaspersson, Karin | Bogstedt, Anna | Jeppsson, Fredrik | Fälting, Johannad | Haeberlein, Samantha Budd | Kugler, Alan R. | Alexander, Robert C. | Cebers, Gvido

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Long Term Memory and Prions

Larry H. Bernstein, MD, FCAP, Curator

LPBI

 

updated 12/12/2015

 

Possible biochemical mechanism underlying long-term memories identified

Why is a prion-like molecular state necessary for persistence of memory? Could a transient memory be made permanent with a “Limitless” NZT-type neurotropic drug — or permanently forgotten?

It’s a nagging question: why do some of our memories fade away, while others last forever? Now scientists at the Stowers Institute for Medical Research have identified a possible biochemical mechanism: a specific synaptic protein called Orb2 can either block or maintain neural synapses (connections between neurons), which create and maintain long-term memories.

So for a memory to persist, the synaptic connections must be kept strong. But how? The researchers previously identified a synaptic protein called CPEB that is responsible for maintaining the strength of such connections in the sea slug (a model organism used in memory research). Recently, they identified a similar protein, called Orb2, in the fruit fly.

Now, using a fruit fly model system, they found that the synaptic connections are kept strong by the transformation of Orb2 from one molecular state to another. And that transformation causes Orb2 molecules to solidify and strengthen the memory connections in the brain.

The authors conclude their paper, published in the current issue of the journal Cell, with several questions. How and what triggers this transformation, how long does it persist? Is the continued presence of a prion-like state necessary for the persistence of memory, and is it correlated with or predictive of long-lasting memory? And most interestingly: can a transient memory about to be forgotten be stabilized by artificial recruitment of the prion-like state (perhaps by a neurotropic compound)?

And what about that ironic link with prions, associated with neurodegenerative disorders? Are prions some twisted form of memory that could one day even have value? We’ll be keeping an eye on where this fascinating research leads.

Technical details: the memory switch

In their latest study, the researchers determined that Orb2 exists in two distinct physical states: monomeric (a single molecule that can bind to other molecules) and oligomeric (a molecular complex).

Like CPEB, oligomeric Orb2 is prion-like — that is, it’s a self-copying cluster. (But unlike prions, oligomeric Orb2 and CPEB are not toxic.) Monomeric Orb2 represses, and oligomeric Orb2 activates a crucial step in the complex cellular process that leads to protein synthesis.

During this crucial step, messenger RNA (mRNA), which is an RNA copy of a gene’s recipe for a protein, is translated by the cell’s ribosome into the sequence of amino acids that will make up a newly synthesized protein. The monomeric form of Orb2 binds to the target mRNA, keeping it in a repressed state.

The Stowers scientists also determined that prion-like Orb2 not only activates translation into amino acids but imparts its translational state to nearby monomer forms of Orb2. As a result, monomeric Orb2 is transformed into prion-like Orb2, so its role in translation switches from repression to activation.

Self-sustaining activation maintains synaptic activity

Stowers Associate Investigator Kausik Si, Ph.D. thinks this switch is the possible mechanism by which fleeting experiences create an enduring memory. “Because of the self-sustaining nature of the prion-like state, this creates a local and self-sustaining translation activation of Orb2-target mRNA, which maintains the changed state of synaptic activity over time,” says Si.

The discovery that the two distinct states of Orb2 have opposing roles in the translation process provides “for the first time a biochemical mechanism of synapse-specific persistent translation and long-lasting memory,” he states.

“To our knowledge, this is the first example of a prion-based protein switch that turns a repressor into an activator,” Si adds. “The recruitment of distinct protein complexes at the non-prion and prion-like forms to create altered activity states indicates the prion-like behavior is in essence a protein conformation-based switch.

“Through this switch, a protein can lose or gain a function that can be maintained over time in the absence of the original stimuli. Although such a possibility has been anticipated prior to this study, there was no direct evidence.”

The research builds upon previous studies by Si and Eric Kandel, M.D., of Columbia University and other scientists. These studies revealed that both short-term and long-term memories are created in synapses.

 

Abstract of Amyloidogenic Oligomerization Transforms Drosophila Orb2 from a Translation Repressor to an Activator

Memories are thought to be formed in response to transient experiences, in part through changes in local protein synthesis at synapses. In Drosophila, the amyloidogenic (prion-like) state of the RNA binding protein Orb2 has been implicated in long-term memory, but how conformational conversion of Orb2 promotes memory formation is unclear. Combining in vitro and in vivo studies, we find that the monomeric form of Orb2 represses translation and removes mRNA poly(A) tails, while the oligomeric form enhances translation and elongates the poly(A) tails and imparts its translational state to the monomer. The CG13928 protein, which binds only to monomeric Orb2, promotes deadenylation, whereas the putative poly(A) binding protein CG4612 promotes oligomeric Orb2-dependent translation. Our data support a model in which monomeric Orb2 keeps target mRNA in a translationally dormant state and experience-dependent conversion to the amyloidogenic state activates translation, resulting in persistent alteration of synaptic activity and stabilization of memory.

 

New Finding on Synapse Destruction May Open Path to Alzheimer’s Therapy

http://www.genengnews.com/gen-news-highlights/new-finding-on-synapse-destruction-may-open-path-to-alzheimer-s-therapy/81252029/

A team led by scientists at the University of New South Wales in Australia say they have discovered how connections between brain cells are destroyed in the early stages of Alzheimer’s disease. They believe their work opens up a new avenue for research on possible treatments for the degenerative brain condition.

“One of the first signs of Alzheimer’s disease is the loss of synapses—the structures that connect neurons in the brain,” noted study leader, Vladimir Sytnyk, Ph.D., of the UNSW School of Biotechnology and Biomolecular Sciences. “Synapses are required for all brain functions, and particularly for learning and forming memories. In Alzheimer’s disease, this loss of synapses occurs very early on, when people still only have mild cognitive impairment, and long before the nerve cells themselves die. We have identified a new molecular mechanism which directly contributes to this synapse loss, a discovery we hope could eventually lead to earlier diagnosis of the disease and new treatments.”

The team studied a specific protein in the brain, neural cell adhesion molecule 2 (NCAM2), one of a family of molecules that physically connects the membranes of synapses and help stabilize these long lasting synaptic contacts between neurons. The researchers paper (“Aβ-dependent reduction of NCAM2-mediated synaptic adhesion contributes to synapse loss in Alzheimer’s disease”) is published in Nature Communications.

Using post-mortem brain tissue from people with and without the condition, they discovered that synaptic NCAM2 levels in the part of the brain known as the hippocampus were low in those with Alzheimer’s disease. They also showed in mice studies and in the laboratory that NCAM2 was broken down by beta-amyloid, which is the main component of the plaques that build up in the brains of people with the disease.

“Our research shows the loss of synapses is linked to the loss of NCAM2 as a result of the toxic effects of beta-amyloid,” pointed out Dr. Sytnyk. “It opens up a new avenue for research on possible treatments that can prevent the destruction of NCAM2 in the brain.”

 

Aβ-dependent reduction of NCAM2-mediated synaptic adhesion contributes to synapse loss in Alzheimer’s disease

Iryna Leshchyns’kaHeng Tai LiewClaire ShepherdGlenda M. HallidayClaire H. StevensYazi D. KeLars M. Ittner & Vladimir Sytnyk
Nature Communications Nov 2015; 6(8836)        doi:10.1038/ncomms9836

Alzheimer’s disease (AD) is characterized by synapse loss due to mechanisms that remain poorly understood. We show that the neural cell adhesion molecule 2 (NCAM2) is enriched in synapses in the human hippocampus. This enrichment is abolished in the hippocampus of AD patients and in brains of mice overexpressing the human amyloid-β (Aβ) precursor protein carrying the pathogenic Swedish mutation. Aβ binds to NCAM2 at the cell surface of cultured hippocampal neurons and induces removal of NCAM2 from synapses. In AD hippocampus, cleavage of the membrane proximal external region of NCAM2 is increased and soluble extracellular fragments of NCAM2 (NCAM2-ED) accumulate. Knockdown of NCAM2 expression or incubation with NCAM2-ED induces disassembly of GluR1-containing glutamatergic synapses in cultured hippocampal neurons. Aβ-dependent disassembly of GluR1-containing synapses is inhibited in neurons overexpressing a cleavage-resistant mutant of NCAM2. Our data indicate that Aβ-dependent disruption of NCAM2 functions in AD hippocampus contributes to synapse loss.

 

Learning and memory processes depend on the number and correct functioning of synapses in the brain. Cell adhesion molecules are enriched in the pre- and postsynaptic membranes. These molecules physically connect synaptic membranes, providing mechanical stabilization of synaptic contacts1, 2, 3, are necessary for the formation of new synapses during neuronal development4, 5, and maintain and regulate synaptic plasticity in adults6, 7, 8, 9, 10.

Alzheimer’s disease (AD) is a neurodegenerative brain condition predominantly of the aging population. One of the earliest signs of AD is the loss of synapses11, which can at least partially be linked to the toxicity mediated by Aβ12, 13, 14, a peptide that accumulates in the brains of AD patients. The impact of AD on synaptic adhesion and the role of synaptic cell adhesion molecules in the progression of the disease remains poorly understood.

The neural cell adhesion molecule 2 (NCAM2), sometimes designated OCAM, belongs to the immunoglobulin superfamily of cell adhesion molecules. NCAM2 participates in homophilic trans-interactions15, 16. During human embryonic development, NCAM2 is expressed in several tissues, including lung, liver, and kidney with the highest expression in the brain17. The expression level of NCAM2 peaks around postnatal day 21 and remains high during adulthood15, suggesting that the protein is necessary both during development and in adult brains. Accordingly, studies with cultured neurons and in NCAM2 deficient mice show that NCAM2 is important for the development of the brain, and the olfactory system in particular18, 19.

The NCAM2 gene is located on chromosome 21 in humans and NCAM2 overexpression has been suggested to be one of the factors contributing to the symptoms of Down syndrome17, which presents with early-onset AD pathology. Single-nucleotide polymorphisms in the NCAM2 gene have been reported as a risk factor related to the progression of AD in the Japanese population20. A recent genome-wide association study has found an association between single-nucleotide polymorphisms in the NCAM2 gene and levels of Aβ in the cerebrospinal fluid in humans, suggesting that NCAM2 is involved in the pathogenic pathway to the senile plaques that concentrate in AD brains21. Since genetic association studies indicate a link between NCAM2 and AD, we have analysed whether AD pathology influences levels of NCAM2 in synapses. Our results indicate that the synaptic adhesion mediated by NCAM2 is highly susceptible to Aβ toxicity and that proteolytic fragments of NCAM2 generated in an Aβ-dependent manner can directly contribute to the induction of synapse disassembly.

 

Synaptic NCAM2 is reduced in the hippocampus in AD

To analyse whether functions of NCAM2 are affected in AD, frozen post-mortem brain tissue of AD patients and non-affected controls (n=10 each) was analysed by western blot with antibodies against NCAM2. The detailed demographic data for the subjects analysed are presented inSupplementary Table 1. Total levels of NCAM2 were slightly increased in the hippocampus, but not significantly affected in the cerebellum or superior temporal cortex in AD (Supplementary Fig. 1). In contrast, levels of VGLUT1, a presynaptic marker-protein of excitatory synapses, were reduced in AD hippocampus (Supplementary Fig. 1), indicating a loss of excitatory synapses. Levels of VGAT, a presynaptic marker-protein of inhibitory synapses, were not significantly affected in any brain region analysed (Supplementary Fig. 1).

Changes in the protein levels in brain homogenates do not necessarily reflect changes in the protein levels in synapses. To analyse whether the synaptic function of NCAM2 is affected in AD, we compared the enrichment of NCAM2 in synaptosomes isolated from the brain tissue of individuals with AD and non-affected controls by western blot analysis of synaptosomes and total homogenates of the brains used for synaptosome preparations. Equal total protein amounts from each probe were applied to the gels to compensate for any possible differences in the yield of synaptosomes because of the synapse loss observed in AD. Western blot analysis with antibodies against actin, VGLUT1, VGAT, synaptophysin (a general presynaptic marker-protein), and PSD95 (a postsynaptic marker-protein), showed that these proteins were enriched to similar levels in synaptosomes from AD and control brains, indicating similar purities of intact synaptosome isolations (Fig. 1a). Western blot analysis showed that in control individuals NCAM2 was highly enriched in synaptosomes from the hippocampus and to a lower degree in synaptosomes from the temporal cortex and cerebellum (Fig. 1a,b). This synaptic enrichment of NCAM2 was significantly reduced in synaptosomes from AD hippocampi (Fig. 1a,b). The synaptic enrichment of NCAM2 was slightly lower in the AD versus control cerebellum, however the difference was not statistically significant (Fig. 1a,b).

 

Figure 1: Synaptic accumulation of NCAM2 is reduced in the hippocampus of AD-affected individuals.

Figure 2: Cleavage of the membrane-adjacent extracellular fragment of NCAM2 is increased in AD brains.

Figure 3: The extracellular domain of NCAM2 binds to Aβ.

Cleavage of NCAM2aa682-701 is increased in AD brains

NCAM2 binds to Aβ in vitro

Figure 4: NCAM2 accumulates in excitatory synapses of cultured hippocampal neurons.

NCAM2 accumulates in excitatory synapses of cultured hippocampal neurons.

http://www.nature.com/ncomms/2015/151127/ncomms9836/images_article/ncomms9836-f4.jpg

(a) Low-magnification image of a cultured hippocampal neuron labelled by indirect immunofluorescence with antibodies against NCAM2, synaptophysin and MAP2. Note expression of NCAM2 along MAP2 positive dendrites. NCAM2 is also expressed in astrocytes (marked a) which are present in these cultures. Scale bar, 20μm. (b) High-magnification image of dendrites of neurons co-labelled with antibodies against NCAM2, synaptophysin and MAP2. Arrows show clusters of NCAM2 partially overlapping with synaptophysin accumulations. NCAM2-negative synapses are also observed (arrowheads). Scale bar, 10μm. (c) High-magnification image of a dendrite of a cultured hippocampal neuron labelled with antibodies against NCAM2, synaptophysin and PSD95. NCAM2 clusters partially overlap with accumulations of PSD95 and synaptophysin (arrows). Scale bar, 10μm. Three-dimensional analysis of the co-localization within the outlined area is on the right. Z-stack has been acquired with 0.15μm steps. The xz and yz sections along the dashed lines on the xy image are shown. Note co-localization of the NCAM2 cluster with synaptic markers. (d) Negative control, that is, labelling performed without primary antibodies, is shown. Scale bar, 10μm.

 

Figure 5: Aβ1–42 oligomers bind to NCAM2 at the cell surface of neurons.

Figure 6: Levels of NCAM2 are reduced in synaptosomes of cultured hippocampal neurons treated with Aβ1-42 oligomers.

Figure 7: NCAM2 co-localizes with Aβ1-42 in brains of APP23 transgenic mice.

Figure 8: NCAM2 binds to Aβ and its synaptic accumulation is reduced in the hippocampus of APP23 transgenic mice.

Aβ removes NCAM2 from synapses of hippocampal neurons

Western blot analysis showed that levels of soluble NCAM2 with the molecular weight of ~100kDa were significantly increased in culture medium from Aβ1-42-treated hippocampal neurons (Fig. 6b), further indicating that Aβ1-42 induces removal of NCAM2 off the neuronal cell surface. In contrast, levels of the soluble proteolytic products of CHL1, another synaptic cell adhesion molecule of the immunoglobulin superfamily26, 27, were not changed in the culture medium from Aβ1-42-treated hippocampal neurons (Fig. 6b). Incubation with Aβ1-42 did not increase levels of soluble NCAM2 in the culture medium from cortical neurons (Fig. 6b), suggesting that cortical neurons are more resistant to Aβ1-42-dependent NCAM2 proteolysis.

Aβ binds to and removes NCAM2 from synapses in APP23 mice

Disruption of NCAM2 adhesion promotes synapse disassembly

Figure 9: Disruption of NCAM2 functions at the neuronal cell surface promotes glutamatergic synapse disassembly.

Disruption of NCAM2 functions at the neuronal cell surface promotes glutamatergic synapse disassembly.

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(ae) Cultured hippocampal neurons were either mock-treated or incubated with the recombinant soluble extracellular domains of NCAM2 (NCAM2-ED), antibodies against the extracellular domain of NCAM2 (NCAM2mAb), or Aβ1-42 oligomers. In a,b, neurons were labelled with antibodies against the extracellular domain of GluR1 before permeabilization of membranes with detergent, and co-labelled with antibodies against synaptophysin after permeabilization of membranes with detergent. Representative images of dendrites are shown (a). Note co-localization of cell surface GluR1 accumulations with synaptophysin clusters in mock-treated neurons, and increased levels of non-synaptic cell surface GluR1 accumulations in neurons treated with NCAM2-ED, NCAM2mAb or Aβ1-42. Graphs (b) show the percentage of synaptic and non-synaptic GluR1 clusters relative to total number of GluR1 clusters along dendrites and numbers of synaptophysin accumulations per dendrite length (mean+s.e.m.). *P<0.0001 (analysis of variance with Dunnett’s multiple comparison test, n>80 dendrites from 20 neurons were analysed in each group). In c, neurons were labelled with antibodies against the extracellular domain of NR1 before permeabilization of membranes with detergent, and co-labelled with antibodies against synaptophysin after permeabilization of membranes with detergent. Graphs show the percentage of synaptic and non-synaptic NR1 clusters relative to total number of NR1 clusters along dendrites (mean+s.e.m.). *P<0.0001 (analysis of variance with Dunnett’s multiple comparison test, n>85 dendrites from 20 neurons were analysed). In d,e, neurons were co-labelled with fluorescent phalloidin and synaptophysin antibodies. Representative images of dendrites are shown in d. Note higher labelling intensity and co-localization with synaptophysin of the phalloidin-labelled polymerized actin accumulations in control neurons versus neurons treated with Aβ1-42, NCAM2-ED or NCAM2mAb. Note increased numbers of filopodia and lamellipodia in neurons treated with Aβ1-42, NCAM2-ED or NCAM2 mAb. Graphs (e) show ratio of the dendrite area-to-length and phalloidin labelling intensity of dendrites of neurons. Mean values+s.e.m. are shown. *P<0.0001 (analysis of variance with Dunnett’s multiple comparison test, n=50 dendrites from 20 neurons were analysed in each group). Scale bar, 10μm (in a,d).

Cleavage-resistant NCAM2 reduces Aβ-dependent synapse loss

Figure 10: Aβ1-42 reduces the number of GluR1-containing synapses in the NCAM2-dependent manner.

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(a) Representative images of dendrites of cultured hippocampal neurons transfected either with control negative miRNA (negative miR) or NCAM2miR and either mock-treated or incubated with Aβ1-42. Transfected neurons were identified by fluorescence of GFP, which is co-expressed together with miRNA. Neurons were co-labelled with antibodies against cell surface GluR1 and synaptophysin. Note that the number of synaptic GluR1 clusters is reduced and the number of non-synaptic GluR1 clusters is increased in neurons transfected with NCAM2miR. Scale bar, 10μm. (b,c) Graphs show mean+s.e.m. percentage of synaptic and non-synaptic GluR1 clusters relative to the total number of GluR1 clusters along dendrites (b) and numbers of synaptophysin accumulations per dendrite length normalized to the mean number in mock-treated neurons (c) for neurons described in (a). (df) Graphs show mean+s.e.m. percentage of synaptic and non-synaptic GluR1 clusters relative to the total number of GluR1 clusters along dendrites (d), number of synaptophysin accumulations per dendrite length normalized to the mean number in mock-treated neurons (e), and area/length ratio (f) in cultured hippocampal neurons transfected either with GFP alone or co-transfected with GFP and non-mutated NCAM2 (NCAM2WT) or NCAM2D693A mutant and either mock-treated or incubated with Aβ1-42. (g,h) Graphs show mean+s.e.m. percentage of non-synaptic GluR1 clusters relative to the total number of GluR1 clusters along dendrites (g) and area/length ratio (h) in cultured hippocampal neurons co-transfected with NCAM2 miR and either GFP, non-mutated NCAM2 (WT) or NCAM2D693A mutant (D693A) and either mock-treated or incubated with Aβ1-42. In bh, *P<0.01 (compared as indicated), ˆP<0.01 (compared with mock-treated neurons transfected with negative miR (b), GFP (df) or co-transfected with NCAM2miR and GFP (gh)), analysis of variance with Tukey’s multiple comparison test, n>50 dendrites from 20 neurons were analysed in each group.

 

Taken together, our results indicate that Aβ affects the numbers of GluR1-containing glutamatergic synapses in a NCAM2-dependent manner.

Alzheimer’s disease is characterized by loss of synapses, which is the strongest correlate of cognitive decline11, 29, 30, 31, 32 and possibly one of the earliest events in AD pathogenesis30, 33. Synapses are long lasting contacts between neurons, which are stabilized by a number of cell adhesion molecules that concentrate in pre- and postsynaptic membranes2, 5. Cell adhesion molecules play an essential role in maintaining synapse functionality and stability. Although cell adhesion molecules of many families are required for the synapse integrity8, 10, elimination of even one type of synaptic cell adhesion molecule is often sufficient to induce abnormalities in synapse ultrastructure and protein composition6, 7. In the present study, we show that levels of the synaptic cell adhesion molecule NCAM2 are markedly reduced in hippocampal synapses in AD brains and Aβ-forming APP23 mice. Our observations that disruption of NCAM2 interactions at the cell surface, knockdown of NCAM2 expression and Aβ exposure result in reduced numbers of glutamatergic synapses in hippocampal neurons suggest that abnormalities in NCAM2-mediated synaptic adhesion contribute to synapse loss in AD.

Although the mechanisms of synapse disassembly in AD remain poorly understood, previous studies indicated that synapse loss can be linked to Aβ-induced toxicity12, 34, 35. Our observations showing that synaptic levels of NCAM2 are similarly reduced in APP23 mice and in cultured hippocampal neurons from wild-type mice exposed to Aβ argue in favour of Aβ-dependent mechanisms in the disruption of NCAM2-mediated synaptic adhesion. We however do not exclude that other factors, such as disrupted trafficking of NCAM2 to synapses, may also contribute to the reduction of NCAM2 levels at synapses. Strikingly, the effects of Aβ on synaptic targeting of NCAM2 were particularly strong in hippocampal but not cortical or cerebellar neurons. The enhanced susceptibility of synaptic NCAM2 to Aβ-dependent proteolysis may therefore contribute to selective vulnerability of the hippocampus to AD.

Our observations that NCAM2 directly interacts with synthetic Aβ1-42, that Aβ1-42 forms a molecular complex with NCAM2 at the neuronal cell surface and that complexes of NCAM2 and oligomers of Aβ can be isolated from APP23 mouse brains, indicate that NCAM2 may function as a previously unrecognized receptor for Aβ at the neuronal cell surface. Previous studies have shown that Aβ can also bind to other cell adhesion molecules at the neuronal cell surface, among which are the prion protein36 and L137. In addition, a number of cell adhesion molecules have been shown to interact with APP, including the neural cell adhesion molecule 1 (NCAM1)38 and TAG1 (ref. 39). It remains to be investigated whether the NCAM2/Aβ complex comprises other adhesion molecules and cell surface proteins. Interestingly, NCAM1, a homologue of NCAM2, binds to prion protein40 and L1 (ref. 41). However, in spite of homology to NCAM2, NCAM1 binds to a region of APP which is different to the Aβ-containing region38.

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Taken together, we show that Aβ induces synaptic loss and proteolysis of NCAM2 in cell culture and APP transgenic mouse models, providing a mechanistic explanation for synaptic NCAM2 changes in AD brains. The detrimental effects of proteolyically cleaved extracellular NCAM2 on synapses may augment the Aβ toxicity in the pathogenesis of AD. The exact molecular mechanisms underlying Aβ-induced NCAM2 changes, and to which degree it contributes to onset and progression of disease remains to be established. Nevertheless, our data reveal a new role of NCAM2 in AD that warrants further investigation.

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