Alpha-synuclein and presynaptic function

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  • NeuroMolecular Medicine 115 Volume 2, 2002

    Alpha-Synuclein and Presynaptic Function

    Implications for Parkinsons Disease

    Simon Lykkebo and Poul Henning Jensen*

    Department of Medical Biochemistry, Aarhus University, DK-8000 Aarhus-C, Denmark

    Received April 17, 2002; Accepted May 16, 2002

    Abstract

    This article focuses on -synucleins role in normal and pathological axonal and presynap-tic functions and its relationship to Parkinsons disease. It is not possible to mention all the con-tributions to aspects of this area. Readers interested in -synucleins relation to aggregation,Lewy lesions, and pathological modifications are referred to the many reviews (see Goldbergand Lansbury 2000; Galvin 2001a; Goedert 2001).

    Index Entries: synuclein; synapse; axon; vesicles; Parkinsons disease; Lewy body.

    NeuroMolecular MedicineCopyright 2002 Humana Press Inc.All rights of any nature whatsoever reserved.ISSN1535-1084/02/02:115129/$20.00

    *Author to whom all correspondence and reprint requests should be addressed. E-mail: phj@biokemi.au.dk

    -Synuclein in Parkinsons Disease

    The small presynaptic protein -synuclein hasreceived great attention since missense mutations inits gene were associated with heritable autosomaldominant early-onset Parkinsons disease in 1997-98(Polymeropoulos, 1997; Krger, 1998). It is a memberof the synuclein gene family, wherein the -, -, and-synucleins, encoded by genes on chromosomes4q21, 5q35 and 10q23, (Campion, 1995; Spillantini,1995; Lavedan, 1998), are predominant in humans(Fig. 1). Synucleins were first identified based on theirpresynaptic nature by expression cloning using asynaptic-vesicle-specific antibody on a vector libraryof mRNA from the electromotor nucleus of the rayTorpedo California (Maroteaux, 1988). Homologous

    mRNAs and proteins have been identified in birdsand mammals, thus demonstrating the highly con-served structure between fish and mammals(Maroteaux, 1988) and birds (George, 1995) (Fig. 1).The initial report coined the term synuclein becauseof its localization to the presynaptic terminus and thenuclear envelope. The latter localization was latershown not to be normal for synucleins.

    Parkinsons disease is the second most commonneurodegenerative diseasethe first is Alzheimersdisease (AD). It is primarily a movement disorder,and is characterized by rigidity, tremor, and bradykinesia, but a wider effect on the nervous system isfrequently noticed, as evidenced by autonomic andcognitive dysfunctions. The definitive diagnosisrelies on the neuropathological demonstration of

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    nerve-cell loss in the pars compacta of the substan-tia nigra in the brainstem, accompanied by the pres-ence of proteinaceous inclusions, Lewy bodies, andLewy neurites. Similar neurodegenerative changesare often found in other parts of the braine.g., thenucleus basalis of Meynert, hypothalamus, cingu-lated gyrus, entorhinal cortex, sympathetic ganglia,and peripheral parasympathetic neurons, wherethey may explain the autonomic and cognitivesymptomatology. For a recent review on clinical andpathophysiological aspects of Parkinsons disease,see (Lang, 1998). The identification of two missense

    mutations in the -synuclein gene on chromo-some 4q21 causing Parkinsons disease representedthe first demonstration of a single gene as causativefor the disease. Both the mutant Ala53Thr in thelarge Contoursi kindred (Polymeropoulos, 1997)and the Ala30Pro mutations in a German family(Krger, 1998) cause an early-onset Parkinsons dis-ease with a dominant heritability of high penetrancethat is indicative of a gain-of-toxic-function by themutant peptide. Mutations in the -synuclein genehave subsequently turned out to be exceedingly rare. The identification of the mutations in the

    Fig. 1. Sequence comparison among synucleins. (A) Alignment of human -synuclein, -synuclein and -synuclein protein sequences. Gray bars indicate the six KTKEGV consensus repeats in the -synuclein sequence.Note the high degree of sequence identity within the N-terminal 62 residues of the gene products as compared totheir divergent C-termini. (B) Alignment of -synuclein protein sequences from human, rat, bird, and fish synu-clein. Note the high degree of conservation of the -synuclein protein in bird and mammals, and even with theunrelated fish synuclein. Dots indicate amino acids identical with the human -synuclein sequence. Gaps areindicated with a wavy line (~). Alignments were performed using the MegAlign software from DNAstar Inc. Acces-sion numbers used for the alignments are: human - (P37840), human - (Q16143), human - (AAC27738), rat - (P37377), bird - (AAA93538), and fish synuclein (A60887).

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    -synuclein gene prompted the immunohistochem-ical examination of brain tissue affected by Parkin-sons disease, and this showed that -synuclein inthe disease is accumulated in the Lewy bodies(Spillantini, 1997). More neurodegenerative diseasesare associated with the development of Lewy bodiesor similar inclusions in which -synuclein is a prominent component, dementia with Lewy bodies,Lewy body variant of AD, motor neuron disease,Hallervorden-Spatz disease (for recent reviews seeDickson, 2001; Goedert, 2001) and multiple systematrophy, where the inclusions occur in the astrocytes(Gai, 1998). Accordingly, these diseases are collec-tively designated as -synucleinopathies. Lewybodies were discovered by Frederic Lewy (Lewy,1912), and were for decades identified histochemi-cally by their affinity for the dye eosin. The sensi-tivity of their detection increased significantly withthe development of immunohistochemistry, forwhich ubiquitin antibodies have represented thestandard for their detection until recently. However,anti--synuclein antibodies have increased the sensitivity even further by demonstrating the exis-tence of subsets of ubiquitin-negative Lewy bodies(McKeith, 1999). Lewy bodies and Lewy neuritescontain large amounts of protein filaments (Forno,1987), and thus resemble the neuropathological

    lesions in other late-onset neurodegenerative dis-eases, such as neuritic tangles in Alzheimers dis-ease and intranuclear inclusions in Huntingtonsdisease. The filaments in Lewy bodies and glialcytoplasmic inclusions contain highly insoluble -synuclein, which is partly modified by proteoly-sis, phosphorylation, and oxidations (Baba, 1998; Gai,1999; Giasson, 2000; Jensen, 2000; Fujiwara, 2002),and they resemble the filaments that can be formedin vitro from recombinant human -synuclein basedon immuno-electronmicroscopical criteria (Conway,1998; Crowther, 1998; Giasson, 1999). This suggeststhat-synuclein is the building block for the core fil-aments that may possess an additional coat of asso-ciated auxiliary cytoplasmic proteins. Structuralanalysis of in vitro-formed filaments have demon-strated that they possess a large content of -foldedstructure and exhibit characteristics of classical amyloid filaments such as the amyloid peptide(A)-filaments in Alzheimers disease (Hashimoto,1998; Conway, 2000a). The accumulation of -synuclein in the Lewy bodies and neurites repre-sents a dramatic relocalization of the protein from itsnormal presynaptic localization (Fig. 2). A less pro-nounced abnormal accumulation of -synuclein and-synuclein in non-Lewy-body structures has alsobeen demonstrated in Parkinsons disease, but no fil-

    Fig. 2. Topological and structural changes in neuronal -synuclein in Parkinsons disease. -synuclein is nor-mally a presynaptic protein. It is synthesized in the cell body and targeted to the nerve terminal by axonal trans-port. A dramatic relocalization from the nerve terminal to the cell body and neurites occurs in the degeneratingneurons during the course of Parkinsons disease. A structural change takes place concomittant with this relocal-ization as -folded amyloid-type -synuclein filaments accumulates in the Lewy bodies and in the neurites. Colorimage available for viewing at www.humanapress.com

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    aments have been detected for these synucleins(Galvin, 1999). Concomitant with the -synucleinrelocalization occurs the above-mentioned structuraltransition of some monomeric unfolded -synucleininto -folded filaments. A biochemical correlate tothe filamentous transition is a decreased solubilityof-synuclein in isolated filaments and in brain tissuecontaining numerous -synuclein-positive inclu-sions (Dickson 1999; Gai 1999; Jensen 2000). A sim-ilar decreased solubility has been demonstrated intransgenic mice expressing human -synuclein anddisplaying pathological symptoms (Kahle, 2001).

    The aggregation of recombinant human -synuclein in vitro represents a nucleation-dependent aggregation process in which filamentformation is preceded by a lag phase where fila-mentogene oligomers build up (Conway, 1998;Wood, 1999). The oligomers represents an ill-definedstructural entity whose existence has been demon-strated biochemically and by atomic forcemicroscopy (Conway, 2000a, 2000b; Rochet, 2000;Volles, 2001). However, the oligomers ability to seedfilament growth emphasizes the importance of fac-tors that increase their concentration. Such factorsmay be subject to dynamic regulation in vivo andcomprise disease-causing mutations, C-terminaltruncation (Crowther, 1998, Jensen, 2000), oxidativemodifications (Hashimoto, 1999; Paik, 2000; Souza,2000), phosphorylations (Fujiwara, 2002), calciumbinding (Nielsen, 2001), and possibly certain -synuclein binding proteins such as synphilin(Engelender, 1999).

    The direct link between -synuclein alterationsand the development of Parkinsons diseaseestablished by the pathogenic mutations in the -synuclein gene, and the demonstration of