Journal of Alzheimer’s Disease 29 (2012) 241–254DOI 10.3233/JAD-2011-111163IOS Press

241

Review

Potential Roles of Peroxisomes inAlzheimer’s Disease and in Dementiaof the Alzheimer’s Type

Gerard Lizarda,∗, Olivier Rouaudb, Jean Demarquoya, Mustapha Cherkaoui-Malkia and Luigi Iulianoc,∗aCentre de Recherche INSERM 866 ‘Lipides, Nutrition, Cancer’, Universite de Bourgogne Equipe ‘Biochimie duperoxysome, Inflammation et Metabolisme Lipidique’, Dijon, FrancebService de Neurologie, CHU de Dijon, Dijon, FrancecDepartment of Medico-Surgical Sciences and Biotechnology, Laboratory of Vascular Biology and MassSpectrometry, Sapienza University of Rome, Latina, Italy

Accepted 1 December 2011

Abstract. In Alzheimer’s disease (AD) and dementia of the Alzheimer’s type (DAT), the role played by peroxisomes is not wellknown. Peroxisomes are present in all eukaryotic cells, with the exception of erythrocytes. They are involved in the �-oxidationprocess of long-chain fatty acids, very-long-chain fatty acids, and branched-chain fatty acids. They participate in the �-oxidationof phytanic acid, the biosynthesis of bile acids, and the breakdown of eicosanoids. Peroxisomes are also involved in the synthesisof specific fatty acids such as docosahexaenoic acid (DHA), which is essential for the brain and retina, and plasmalogens (PLGN),which play crucial roles in neural cells and are essential components of myelin. Several studies conducted in animal modelsand in humans provided evidence for a role of DHA in preventing brain degeneration. Significantly lower levels of PLGN wereobserved in patients with severe dementia. Moreover, a decreased activity of carnitine acetyltransferase, an enzyme present inperoxisome (but also detected in mitochondria, endoplasmic reticulum, and nucleus), was reported in AD patients. We give anoverview of the potential role of peroxisomes, especially in the part played by DHA, PLGN, carnitine, and carnitine-dependentperoxisomal enzymes, on the development of AD and DAT. The potential of developing novel therapies targeted on peroxisomalmetabolism to prevent cognitive decline and other age-related neurological disorders is discussed.

Keywords: Alzheimer’s disease, carnitine-dependent enzymes, dementia, DHA, peroxisome, plasmalogen

∗Correspondence to: Dr. Gerard Lizard, Ph.D., Centre deRecherche INSERM 866 – Equipe BIO-peroxIL, Faculte des Sci-ences Gabriel, 6 Bd Gabriel, 21000 Dijon, France. Tel.: +33380 39 62 56; Fax: +33 380 39 62 50; E-mail: Gerard.Lizard@u-bourgogne.fr and Prof. Luigi Iuliano, MD, Department of Medico-Surgical Sciences and Biotechnology, Laboratory of Vascular Biol-ogy and Mass Spectrometry, Sapienza University of Rome, corsodella Republica 79, 04100 Latina, Italy. Tel.: +39 0773 31757231;Fax: +39 06 62 29 1089; E-mail: luigi.iuliano@uniroma1.it.

ALZHEIMER’S DISEASE AND DEMENTIAOF THE ALZHEIMER’S TYPE:CLINICOPATHOLOGICAL FEATURES

Aging is associated with enhanced susceptibilityto brain dysfunction, loss of memory, and cognitivedecline, which significantly affect the quality of lifeof affected individuals. Neuropathological and cogni-tive changes associated with dementia syndromes areprogressive, interrelated, and highly complex. Amongthese catastrophic neurologic disorders, Alzheimer’s

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242 G. Lizard et al. / Peroxisomes and Dementia

Fig. 1. Biomarkers of brain lesions in patients with AD or with dementia of the Alzheimer’s type. A) Amyloid plaque pathology (senileplaque). Monoclonal antibodies were used to detect phosphorylated tau proteins. B) Bielschowsky-Hirano’s silver staining was used to identifyneurofibrillary tangles. C) Magnetic resonance imaging T2 Flair (coronal view) shows atrophy of the right medial temporal lobe in a patientwith mild AD. D) Positron emission tomography with 18F-fluorodeoxyglucose reveals hypometabolism of the medial temporal lobe and theposterior associative cortex in a patient with mild AD.

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disease (AD) represents the most prevalent neurologi-cal dysfunction in the elderly.

AD is a clinicopathological entity centered on thepresence of a progressive dementia, which includesepisodic memory impairment and the involvement ofother cognitive domains or skills, and specific neu-ropathological changes, which include neurofibrillarytangles and senile plaques (Fig. 1, A–B) often associ-ated with synaptic loss and vascular amyloid deposits[1]. At the earliest stages of typical AD, episodic mem-ory disorder is the main symptom, due to the initiallocalization of neurofibrillary tangles in the medialtemporal lobe. During AD evolution, other domainsare progressively involved, including executive func-tion, language, praxis, and complex visual processingand gnosis. Currently, there is an increasing demandfor non-invasive imaging and biological tools for thediagnosis of AD, which is still based on clinical symp-toms [2]. New criteria have recently been proposed toidentify a prodromal form of the disease using imagingtechniques, including magnetic resonance and positronemission tomography, and specific biomarkers in thecerebrospinal fluid [3]. A typical magnetic resonanceimaging feature in AD is atrophy of the medial tempo-ral lobe (Fig. 1C), which has been reported in 71–96%of patients, depending on disease severity. Positronemission tomography is used to measure regionalbrain metabolism by 18F-deoxyglucose. A reductionin glucose metabolism in bilateral temporal parietalregions and the posterior cingulate has been proposedas diagnostic criteria for AD (Fig. 1D), with sensitivityand specificity of 88–95% and 62–74%, respectively[4].

Specific biomarkers of AD include amyloid-� 1-42(A�42), total tau (t-tau), and phospho-tau (p-tau).In AD, the concentration of A�42 in cerebrospinalfluid is decreased while that of t-tau is increasedcompared to the concentrations in healthy controls.A number of other biological markers have poten-tial applications in AD, including those associatedwith lipid metabolism, such as oxysterols [5–7].As the levels of some lipids, including plasmalo-gens (PLGN), docosahexaenoic acid (DHA), andvery long chain fatty acids (C22:0, C24:0, C26:0)are altered in the brain of patients suffering fromAD and dementia of the Alzheimer’s type (DAT)[8–10], and, as the metabolism of these compoundsis relied with peroxisomal metabolism, a poten-tial role of peroxisome can be suspected in theseage-related neurodegenerative diseases. Some studiesand arguments supporting this hypothesis are pre-sented.

PEROXISOMES: BIOGENESIS ANDBIOCHEMICAL PATHWAYS

Peroxisomes are DNA-free organelles present inalmost all eukaryotic cells including unicellulareukaryotes and higher plant cells [11, 12]. Peroxisomesare morphologically characterized by a single limitingmembrane, a finely granular matrix and a size rangeof 0.1 to 1 �m in diameter [13] (Fig. 2). Peroxisomebiogenesis is still a poorly characterized process. Ithas been suggested that peroxisome could arise fromeither the endoplasmic reticulum or by the growthand division of pre-existing peroxisomes [14, 15].As peroxisomes are DNA-free, they require specificprotein import pathways. These include two classesof matrix-targeting signals for peroxisomal proteins,namely peroxisomal targeting signal 1 and 2 (PTS1and PTS2), and their respective cytosolic receptors per-oxin 5 and 7 (PEX5p and PEX7p), respectively. Thereceptor-cargo complex translocates into the peroxi-some where the complex is dissociated: the cargo isreleased into the peroxisomal matrix, while the recep-tor is recycled back to the cytosol [16]. Besides thesetwo pathways for PTS1 and PTS2, several matrix pro-teins are imported in a PEX5p-dependent manner,despite the fact that they lack a typical PTS1 [15].As these proteins also lack a PTS2, they are desig-nated as non-PTS proteins [17]. For some of theseproteins, it has been reported that they form complexeswith PTS-containing proteins and, therefore, enter theperoxisomes by a “piggyback” mechanism [18].

Currently, more than 50 enzymes involved in var-ious metabolic pathways have been identified inmammalian peroxisomes. The main metabolic path-ways associated with peroxisomes are summarized inFig. 3 [19, 20]. Peroxisomes are involved in somecellular functions such as hydrogen peroxide detox-ification mediated by catalase, in the degradationof purines, polyamines, and eicosanoids, and theyalso contribute to lipid metabolism [19–21]. Thus,peroxisomes are necessary for the �-oxidation of very-long-chain fatty acids (VLCFA) such as C24:0 andC26:0, the 2-methyl-branched fatty acid pristanic acid(2,6,10,14-tetramethylpentodecanoic acid), the inter-mediates of bile acid synthesis di-hydrocholestanoicacid (DHCA) and tri-hydrocholestanoic acid (THCA),and the �-oxidation of phytanic acid as well [13]. Theperoxisomal �-oxidation pathway has the same basicstructure as found in mitochondria and is made up offour subsequent steps of dehydrogenation, hydratation,second dehydrogenation, and thiolytic cleavage. In thisprocess, the fatty acid undergoes successive rounds

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Fig. 2. Identification of peroxisomes by fluorescence microscopyand transmission electron microscopy. A) Fluorescence microscopyof peroxisomes in 158N murine oligodendrocytes. The cellswere stained with a rabbit polyclonal antibody raised againstthe peroxisomal transporter ABCD3 (PMP70), and the nuclei(blue) were counterstained with Hoechst 33342. Note the uniformintracellular distribution of peroxisomes (green). Peroxisomes ultra-structure in mouse hepatoma cells (B) and in 158N cells [128,129] (C). Diaminobenzidine-cytochemical localization of catalaseinside the peroxisomes, the cells were incubated in alkaline 3,3-diaminobenzidine medium followed by post-fixation in 1% aqueousosmium tetroxide and 2% uranyl acetate [130–132]. Black arrowspoint towards peroxisomes; white arrows point towards mitochon-dria.

of 2-carbons chain-shortening. In order to catalyse�-oxidation, peroxisomes are equipped with a full com-plement of enzymatic machinery, including acyl-CoAoxidases (ACOX), bifunctional proteins (BP), and per-oxisomal thiolases (PTH). The first step of �-oxidationis catalysed by diverse ACOX isoforms, includingACOX1, ACOX2, and, depending on species/tissue,ACOX3; two bifunctional protein isoforms, LBP andDBP, catalyzes the second and third steps; while thelast step is handled by the two peroxisomal thiolasesPTH1 and PTH2/SCPx [19–22]. DPB is also the main,if not the only, enzyme involved in the �-oxidation ofVLCFAs, pristanic acid, DHCA, and THCA.

Interestingly, the oxidation of C24:6, n3, the pre-cursor of DHA (C22:6, n3), involves the same set ofenzymes as required for the oxidation of C26:0 [19, 20,22]. Moreover, two peroxisomal carnitine acyltrans-ferases catalyse the exchange of acyl groups betweencarnitine and coenzyme A (CoA) and contribute tosubstrate channeling between peroxisomes and mito-chondria [23, 24]. While carnitine octanoyltransferase(COT) is localized only in the peroxisomal matrix,carnitine acetyltransferase (CAT) has been reportedin several cell compartments, including, endoplas-mic reticulum, mitochondria, nucleus and peroxisomes[24].

The important roles of peroxisomes in humanhealth became obvious when some peroxisomal abnor-malities were identified in severe neurodegenerativeand demyelinating brain diseases [25]. Zellwegersyndrome and rhizomelic chondrodysplasia punc-tata lack of peroxisome synthesis [26]. X-linkedadrenoleukodystrophy [27, 28] and pseudo-neonataladrenoleukodystrophy [29] are associated with a defi-ciency in the ABCD1 transporter and Acyl-CoAoxidase 1, respectively. It is now well establishedthat these organelles play a crucial role in neural cellgrowth, brain development and retina function throughtheir implication in the synthesis of DHA and PLGN,which are essential components of myelin [30].

In addition, regarding the decline of peroxisomalfunctions capacity with age [31–33], peroxisomaldysfunctions associated with aging might favor neu-rodegenerative diseases, including AD [8, 34–37]. Thefirst evidence of the potential role of peroxisomes in thedevelopment of AD was established in vitro on primaryrat hippocampal neuron cultures. In these cells, perox-isomal proliferation, induced by Wy-14.463, which isa potent PPAR� agonist, was shown to protect againstcell death induced by the A� peptide [35]. Moreover,in vivo in the Tg2576 mouse model of AD at the ageof 3 months, when no apparent neuroanatomical or

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Fig. 3. Major peroxisomal metabolic activities and potentially affected metabolic pathways in Alzheimer’s disease (AD) and in dementia of theAlzheimer’s type (DAT). Peroxisomes are single membrane-bound organelles with a size range of 0.1 to 1 �m in diameter and containing a finelygranular matrix. Peroxisomes are virtually present in all mammalian cells with multiple metabolic pathways, including both biosynthesis (i.e.,plasmalogen (PLGN)/ether phospholipid biosynthesis, docosahexaenoic acid (DHA; C22:6, n3), and bile acids biosynthesis) and degradation(i.e., fatty acid �-oxidation and �-oxidation; degradation of eicosanoids) pathways (see references [19–23] for detailed descriptions of thesedifferent pathways). Metabolic pathways leading to PLGN and DHA synthesis potentially affected in AD and DAT are in yellow. CoA: coenzymeA; CAT: carnitine acetyltransferase; COT: carnitine octanoyltransferase; DHAP: dihydroxyacetone phosphate; DHAPAT: dihydroxyacetonephosphate acyltransferase; DHCA: di-hydroxycholestanoic acid; FA: fatty acid; THCA: tri-hydroxycholestanoic acid; VLCFA: very-long-chainfatty acid.

cytological signs of the disease are apparent, signifi-cant peroxisome alterations were observed [38]. It isnoteworthy that substantial peroxisome-related alter-ations, which may contribute to the progression of ADpathology, were reported on postmortem brain tissuesof patients with AD [9].

In AD and DAT, some studies support that potentialperoxisomal alterations would mainly concern DHAand PLGN as the synthesis of these compounds occurs,at least in part, in the peroxisome (Fig. 3).

EVIDENCE OF THE INVOLVEMENT OFDHA AND ITS DERIVATIVES IN THEDEVELOPMENT OF AD AND DAT

DHA pertains to the fatty acid class, which is oneof the most complex categories of lipids. Fatty acids, astructurally linear chain of carbon atoms, are classified

regarding the number of carbon atoms and the numberof double bonds. DHA is defined as a (C22:6, n3) fattyacid because its structure is composed of 22 carbonatoms and six double bonds; n3 stands for the positionof the first double bond starting from the methyl termi-nal. DHA can be taken from the diet or synthetized inthe liver from linolenic acid (C18:3, n3), an essentialfatty acid [39, 40]. To date, it is known that hepato-cytes are able to synthetize DHA from linolenate usingsuccessive elongation and desaturation reactions [22].Then, DHA is esterified into phospholipids, secretedas lipoproteins, and delivered to the brain and retina[10]. In this cascade of events leading to DHA synthe-sis, the role played by peroxisome, in association withthe endoplasmic reticulum, is primordial (Fig. 4).

DHA functions directly at the cellular level, orit might act through its transformation products.Cyclopentanone neuroprostanes are formed by the

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Fig. 4. Metabolic pathway leading to docosahexaenoic acid (DHA) synthesis. DHA (C22:6, n3) is an important fatty acid that can be eithersynthesized de novo or obtained from the diet. DHA can be obtained in vivo from its parent compound, the essential fatty acid linolenic acid(C18:3, n3) through enzymatic pathways occurring in the endoplasmic reticulum (ER) and peroxisomes. In the endoplasmic reticulum, linolenate-CoA (C18:3, n3)-CoA is converted to (C24:6, n3)-CoA through sequential reactions of �6-desaturase and elongases. (C24:6, n3)-CoA is thentranslocated in the peroxisome to be shortened by cycles of �-oxidation. This process is made up of four subsequent steps of dehydrogenation,hydratation, second dehydrogenation, and thiolytic cleavage that successively involve acyl-CoA oxidase1 (ACOX1), D-bifunctional protein(DBP), and thiolase (in humans, either 3-oxoacyl-CoA thiolase encoded by ACAA1 or SCP-oxoacyl-CoA thiolase (SCPx) encoded by sterolcarrier protein 2 (SCP2), have been described [19, 20, 22]). The first round of �-oxidation generates DHA that is transferred into the endoplasmicreticulum, where it is eventually esterified to phospholipids and transported to other cellular or tissue compartments. The metabolism of DHAincludes its biotransformation by 15-lipoxigenase (15-LPO) to the hydroxy derivative 10,17-dihydroxy-docosa-4,7,11,13,15,19-hexaenoic acid(NPD-1). In addition, DHA is highly susceptible to free radical oxidation to give neuroprostanes. The main neuroprostane nPF4�-VI can be usedas a biomarker of DHA peroxidation in the brain but not in urine. Neuroprostane nPF4�-VI has been shown to be �-oxidized in the mitochondriato iPF3�-VI, the prototypical free radical oxidation product of the fatty acid eicosapentanoate (EPA). Urinary iPF3�-VI has been suggestedas a biomarker of both EPA and DHA peroxidation in urine [66]. 1) peroxisomal �-oxidation; 2) �6-desaturase; 3) elongase; 4) 15-LPO; 5)mitochondrial �-oxidation. EPA, eicosapentaenoic acid; DPA, docosapentanoic acid; PL, phospholipids; TH, thiolase.

non-enzymatic autoxidation of DHA [41, 42], and theyare found in human and rodent brain tissues [42]. Neu-roprostanes belong to the family of non-enzymaticprostaglandin-like compounds produced by free rad-ical oxidation of polyunsaturated fatty acids (PUFAs),of which the most studied are isoprostanes derivedfrom the autoxidation of arachidonic acid [43, 44].

An additional DHA transformation product is10,17-dihydroxy-docosa-4,7,11,13,15,19-hexaenoicacid, namely neuroprotectin D1 (NPD-1) [45]. AfterDHA is cleaved from membrane phospholipidsby phospholipase A2, induced as a response tooxidative stress and/or neurotrophin activation [46],

stereospecific oxygenated derivatives of DHA arecreated through the action of 15-lipoxygenase on freeDHA, generating NPD-1 that elicits potent neural andretinal protective effects [10, 47].

The benefits of n3 series fatty acids in AD and DATare supported by several laboratory and clinical studies.Animals fed a diet enriched with n3 PUFAs, includingDHA, have better regulation of neuronal membraneexcitability [48, 49], increased levels of neurotransmit-ters and a higher density of neurotransmitter membranereceptors [50], decreased levels of lipid peroxides [51],and higher levels of antioxidant enzymes [52]. In thiscontext, a potential stimulation of catalase by DHA

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cannot be excluded [53]. Animals fed diets enrichedwith n3 fatty acids including DHA were also found tohave superior learning acquisition and memory perfor-mance over the animals fed control diets [54, 55].

An early-onset AD transgenic mouse model, car-rying the double-mutant form of human amyloid-�protein precursor (A�PP), was found to develop lowerlevels of A� and amyloid plaques in the brain afterbeing fed a low fat diet enriched in DHA com-pared to its control littermates [56, 57]. The proposedmechanisms of n3 fatty acids (including DHA) sup-port the theory that they can influence amyloidogenicprocessing through several distinct and interrelatedmechanisms: 1) facilitation of the interaction of�-secretase with A�PP to produce non-toxic fragmentsand preventing the formation of A�; 2) shielding ofthe essential recognition sequence and intra-membranecleavage site of γ-secretase; 3) serving as a local sinkfor free radicals that reduce the enzymatic augmen-tation of γ-secretase activity, which can be inducedby free radical damage to the protein complex thatis important for the regulation of normal γ-secretasefunctioning; and 4) directly inhibiting fibrillation andthe formation of toxic oligomeric species of A� [58,59]. In addition, it has also been reported that DHAcan inhibit c-Jun N-terminal kinase and phosphoryla-tion of the adaptor protein insulin receptor substrate-1and the tau protein (which is a marker of brain degen-eration) [60, 61] in cultured hippocampal rat neurons[62]. Dietary supplementation with DHA in the 3×Tg-AD mouse model of AD also reduced the intraneuronalaccumulation of both A� and the tau protein [63].

Currently, numerous methods are available forthe measurement of DHA and its derivatives. Themost affordable methods are based on gas chro-matographic separation of DHA methyl esters andmeasurements with a flame ionization detector. Sam-ples are first subjected to direct transmethylation,which overcomes the hydrolysis step to release DHAesterified to other lipids [64, 65]. For high-sensitivityand high-throughput analyses of DHA, mass spec-trometry methods are available for quantitativemeasurement thanks to the availability of deuter-ated standards. Methods for measuring neuroprostanesbased on gas chromatography/mass spectrometry andliquid chromatography-tandem mass spectrometry(LC-MS/MS) have been reported [41, 42, 66]. NPD-1is available in its deuterated form and thus can bequantitatively measured by LC/MS [67, 68].

Altogether, whereas long-term intervention studieson individuals with cognitive reductions are awaited todefine the benefit of DHA [69–71], some observations

still support the hypothesis that the down-regulationof peroxisomal functions occurring during aging [33]might affect the metabolic pathway involved in DHAsynthesis, and favor decreasing cognition and increas-ing neurodegeneration [72–74].

EVIDENCES OF PLGN MODULATION INAD AND DAT

Plasmalogens (1-O-alk-l′-enyl-2-acyl glycerophos-pholipids) constitute a special class of phospholipidsthat are characterized by the presence of a vinylether bond at the sn-1 position [30]. PLGN, whichare partly synthesized in peroxisomes, have a widelydiffering distribution throughout tissues. They arecomposed of a glycerol backbone with one carbonlinked to phosphocholine or phosphoethanolamine,giving two species: choline-PLGN and ethanolamine-PLGN, respectively (Fig. 5). Grey matter is moreenriched by ethanolamine-PLGN (22.4%) comparedto choline-PLGN (0.9%) [75].

In the PLGN, the sn-2 carbon is linked to a PUFA,usually linoleic acid, arachidonic acid or DHA, and thesn-1 carbon is linked to a saturated or monounsaturatedfatty alcohol, which is almost exclusively hexade-canol (C16:0), octadecanol (C18:0) or �9-octadecanol(C18:1) [76] (Fig. 5).

In the white matter, the PLGN is predominantlycomposed of C18:1, C20:1, and C22:4 fatty acids,whereas in the grey matter the PLGN-fatty acidsare mainly C20:4, C22:4, C20:4, and C22:6 [77].Choline-PLGN and ethanolamine-PLGN have each ashort lifetime of about 30 and 180 min, respectively[78]. These PLGNs are selectively hydrolyzed by aPLGN-specific phospholipase A2 (psPLA2), whichreleases the PUFA and lyso-PLGN [30, 79]. ThePLGN can be regenerated from lyso-PLGN via re-acylation. A further degradation step is controlled bylyso-plasmalogenase (lyso-PLGNase), which releasesthe fatty alcohol in the aldehyde form and the glyc-erophosphate backbone [30, 79].

The first two steps in PLGN biosynthesis takeplaces in the peroxisomes [19, 20, 30]. The firststep in the biosynthesis of PLGN involves the ester-ification of dihydroxyacetone phosphate (DHAP)with a long-chain acyl-CoA ester and is carriedout by dihydroxyacetone phosphate acyltransferase(DHAP-AT), leading to the formation of 1-acyl-DHAP. In the second step, the characteristic etherbond at the sn-1 position of PLGN is introducedby the replacement of the sn-1 fatty acid with a

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Fig. 5. Plasmalogen turnover. Plasmalogens (PLGN) are a group of ether-phospholipids with a widely differing distribution throughout tissues.They are composed of a glycerol phosphate backbone with the phosphate group in the sn-3 position linked to a choline or an ethanolamineresidue (R), which respectively result in choline-PLGN (GP-C) and ethanolamine-PLGN (GP-E). The sn-2 position bounds with an ester linkagea polyunsaturated fatty acid (PUFA), which is usually linoleic acid (LA), arachidonic acid (AA), or docosahexaenoic acid (DHA). The sn-1position bounds a long carbon chain in a vinyl-ether linkage. That carbon chain (R1) is the fatty alcohol (Falc) hexadecanol (C16:0), octadecanol(C18:0), or �9-octadecanol (C18:1), which are alcohol counterparts of the saturated fatty acids palmitic (C16:0) and stearic acid (C18:0), orthe monounsaturated fatty acid oleic acid (C18:1). Phospholipase A2 (psPLA2) hydrolyses specifically PLGNs delivering the PUFA mojety,which may act as lipid messanger, and lyso-plasmalogen (lyso-PLGN). The latter can be re-acylated for the regeneration of PLGNs. Otherwise,lyso-plasmalogenase (lyso-PLGNase) makes PLGN degradation to releasing the long carbon chain from sn-1 as fatty aldehyde (Fald) and theglycerol phosphate backbone.

long-chain fatty alcohol. This reaction is catalyzedby alkyl-dihydroxyacetone phosphate synthase, withalkyl-DHAP as the product. The process of PLGNbiosynthesis, which is initiated in the peroxisome,is completed in the endoplasmic reticulum. Actually,the enzymes DHAP and DHAP-AT are only foundin peroxisomes whereas acyl/alkyl-dihydroxyacetonephosphate reductase, which catalyses the reductionof the ketone group at the sn-2 position of alkyl-DHAP, has been co-localized in both peroxisomesand endoplasmic reticulum [80]. The latter containsthe remaining enzymes involved in PGLNs synthe-sis [30]. Although PLGNs are structural membranecomponents and a reservoir for second messengers,they might also be involved in membrane fusion, ion

transport, and cholesterol efflux. Furthermore, theycould also act as antioxidants, thus protecting cellsfrom oxidative stress [34]. In support of the antioxidantfunction is the finding that oxidative stress, trigger-ing an unfolded protein response in the endoplasmicreticulum, was reported in the Pex2 null mouse modelfor peroxisomal biogenesis disorders [81]. PLGN-deficient cells exhibit altered lipid metabolism at thelevel of cholesterol transport, vesicular fusion, andtransmembrane protein function [82]. Reduced lev-els of PLGN are also associated with impaired Ca2+release, intracellular Ca2+ overload, and endoplasmicreticulum stress [83, 84].

Currently, several methods are available for themeasurement of PLGN, such as those based on

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radioactivity tracers [85], which take advantage of theability of iodine to react with the vinyl-ether linkagethat is characteristic of PLGN. PLGN are measuredby quantifying the amount of lyso-PLGN or long-chain fatty aldehydes derived from the cleavage ofthe specific vinyl-ether linkage at the sn-1 position[86]. Spiteller and co-workers developed proceduresbased on derivatization reactions usable in gas chro-matography and mass spectrometry or flame ionizationdetection [87–89]. The complexity of the PLGN classrequires a lipidomic approach for analyzing thesecompounds in biological samples. The expansion ofLC-MS technology has provided the opportunity toquantitatively measure PLGN in high-throughput set-tings. Recently, Goodenowe and colleagues validated amethod for screening eight ethanolmine-PLGN classesby atmospheric pressure-chemical ionization massspectrometry in plasma [8].

Interestingly, in postmortem brain samples frompatients with AD, a significant and selectivedeficiency of ethanolamine-PLGN relative to phos-phatidylethanolamine was identified. This lipid defectshowed an anatomical specificity, being more markedat the site of neurodegeneration in the AD brain than ina region relatively spared by the disease (mid-temporalcortex versus cerebellum) [90]. Thus, in patients withAD or in patients with DAT, it was reported thatPLGN levels, as measured by electrospray ioniza-tion mass spectrometry or LC-MS/MS, were depletedin the cortex and hippocampus [91–92]. Disturbedcholine PLGN and phospholipid fatty acid concentra-tions in the prefrontal cortex of an AD patient were alsoobserved [93]. In addition, circulating levels of PLGNwere also found to be significantly decreased in serumfrom clinically and pathologically diagnosed DAT sub-jects at all stages of dementia, and the severity of thisdecrease correlated with the severity of dementia [8,94]. Interestingly, this decrease in PLGN levels couldbe due to a new physiological function of the A�PPin PLGN metabolism. The intracellular domain ofA�PP was found, both in vivo and in vitro, to increasethe expression of alkyl-dihydroxyacetonephosphate-synthase, a rate-limiting enzyme in PLGN synthesis[95]. It was also reported that in AD patients, phos-phatidylethanolamine PLGN was the only lipid toexhibit major structural modifications, a significantdecrease in polyunsaturated fatty acids and oleic acidand a shift of the aldehyde pattern from C18:1 toC18:0 [96]. It has been hypothesized that this decreasein PLGN in the brain lesions of patients with ADor DAT may be due to the stimulation of PLGN-selective phospholipase A2 or to the decreased activity

of PLGN-synthesizing enzymes that occur in peroxi-somes [34].

ROLES OF CARNITINE, CAT, AND COT INAD AND DAT

Acetyl-L-carnitine (ALCAR) is present in high con-centrations in the brain [97], and it can be formed inthe body or obtained through foods and can cross theblood-brain barrier [98, 99]. Several reports indicatethat ALCAR might be involved in synapse functions[100–103], cholinergic neural transmission [97], andmitochondrial metabolism of neurons [104]. Alto-gether, these data suggest that ALCAR may play arole in protection during AD progression, especiallyin terms of neurotoxicity.

In the cells, ALCAR synthesis is mediated byenzymes that use L-carnitine and acyl-CoA esters assubstrate. In this reaction, the acyl moiety is trans-ferred from CoA-ester to L-carnitine forming ALCARand unsterified CoA [23, 24]. The reaction, which isreversible, is catalyzed by enzymes of the carnitineacyltransferase family [105], a group of widely dis-tributed proteins in the cells, and especially by CATand in a less extends by COT. COT is located in theperoxisome and is putatively involved in the exportof medium-chain fatty acids out of the peroxisome.Although its mechanisms of neuroprotection are stillnot well known, ALCAR may act as a precursor foracetylcholine synthesis by favoring the acetylationof choline or by activating/inducing choline acetyl-trasferase [106].

In APOE4 transgenic mice, as a model of AD,ALCAR has been shown to prevent age-related mito-chondrial alteration in hippocampal neurons [107]. Ithas been reported that ALCAR improves cognitivefunctions and behavioral symptoms in patients withAD and DAT [108–112], and it may slow the progres-sion of AD in younger subjects [113]. Currently, onlyfew data are available on CAT and COT activities inpatients with AD or DAT. Compared to normal brains,unchanged or decreased global CAT activities havebeen shown in the brain of AD patients [114–116],whereas no modification of COT activity has beendescribed to date [116].

PEROXISOMES: A POTENTIAL TARGETFOR THE TREATMENT OF AD AND DAT

In addition to the conventional treatments that areactually used to treat AD and DAT, some additional

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therapies capable of counteracting peroxisomal defi-ciencies can be envisaged. Thus, new therapies that acton peroxisomal metabolism could potentially be devel-oped to prevent cognitive decline and other age-relatedneurological disorders.

The ability to counteract DHA deficiency is easyto perform via diet supplementation in animal mod-els [55, 117]. However, in humans, current evidencefor better cognition under treatment by DHA is notconvincing, and further long-term intervention studieson individuals with cognitive reductions are awaited[69, 70]. A better understanding of the relationship ofDHA intake with the distribution of DHA in tissues istherefore essential and might help to circumvent thisproblem. An additional strategy, alternative to supple-mentation, aimed at favoring DHA accumulation inbrain cells could be targeted at peroxisome level bystimulating the metabolic pathway that contributes toDHA synthesis. This pharmacologic strategy, whichcould not only contribute to an increase in DHA lev-els but also NPD-1 signaling, could be expected tobe beneficial to brain cells in reducing brain cell dys-functions and to favor neural network functions ofthe brain including memory and cognition [118]. Itshould be emphasized, however, that the intracellularincrease in DHA is potentially deleterious, given theoxidizability of this highly unsaturated fatty acid, withredirection of its metabolism to the neuroprostanespathway. Thus, intracellular DHA level should takeinto account the inhibition of its degradation by oxida-tive stress-mediated mechanisms. As the tissues ofinterest (brain, retina) are generally inaccessible forfatty acid analysis in humans, and as DHA levelsare thus difficult to determine inside the cells ofthese tissues, the identification of surrogate biomark-ers will be also required for defining DHA status andits benefits in patients with AD or DAT [119, 120].The monitoring of DHA-derived neuroprostanes inurine, which could be useful as non-invasive mark-ers in AD, failed to demonstrate detectable levels[121]. Lawson et al. provided evidence showing thatneuroprostanes undergo �-oxidation and are trans-formed into class 3 isoprostanes, which belong theclass derived from eicosapentaenoic acid (EPA) self-oxidation [66]. Interestingly, the iPF3�-VI moleculecould be a non-invasive marker in urine for measuringthe combined endogenous self-oxidation of EPA andDHA, and might be relevant in syndromes of neurode-generation. In addition, it has also been reported thatthe rate-limiting enzymes involved in PLGN synthe-sis, and which are present in the peroxisome, mightbe upregulated by DHA in brain cells [122]. Similarly,

catalase activity can be upregulated by DHA in C6 ratglioma cells [123]. Therefore, peroxisomal enzymesregulating PLGN synthesis and catalase, which is aspecific peroxisomal enzyme, might also constitutenew potential therapeutic targets for preventing neu-rodegeneration.

CONCLUSION AND PERSPECTIVES

Mammalian cells contain different types of cellularorganelles—Golgi apparatus, lysosomes, mitochon-dria, and peroxisomes—that have highly specializedfunctions. Mitochondria and peroxisomes are tightlyconnected in certain metabolic pathways [124]. Per-oxisomes play important roles in the central nervoussystem, where they are essential for myelination [125,126] and the synthesis of DHA and PLGN, which areimportant for the functioning of neural cells [34, 127].Noteworthy, quantitative and/or qualitative abnormal-ities of these compounds have been suggested to playa role in the development of AD or DAT [56, 57, 92,93]. Therefore, it is tempting to speculate that dys-functions in peroxisomal metabolism could contributeto the development of neurodegenerative diseases.Consequently, a better understanding of the role of per-oxisomes played in the pathophysiopathology of ADand DAT could contribute to a better knowledge of thegenesis of these diseases and also may contribute toidentifying new therapeutic targets and treatments.

ACKNOWLEDGMENTS

This work was supported by grants from theINSERM, the Universite de Bourgogne, and the Con-seil Regional de Bourgogne and Sapienza Universityof Rome.

Authors’ disclosures available online (http://www.j-alz.com/disclosures/view.php?id=1087).

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