- Email: [email protected]
Pathobiological features in neurodegenerative diseases: an overview
International Congress Series 1260 (2004) 69 – 75
www.ics-elsevier.com
Pathobiological features in neurodegenerative diseases: an overview Makoto Higuchi a,c,*, John Q. Trojanowski b, Virginia M.-Y. Lee a a
Center for Neurodegenerative Disease Research, University of Pennsylvania, Philadelphia, PA 19104, USA b Institute on Aging, University of Pennsylvania, Philadelphia, PA 19104, USA c Laboratory for Proteolytic Neuroscience, Riken Brain Science Institute, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan Received 20 July 2003; received in revised form 29 July 2003; accepted 4 September 2003
Abstract. Discovery and characterization of fibrillary aggregates composed of specific neuronal/glial proteins in Alzheimer’s disease (AD) and other neurodegenerative diseases have not only provided molecular insights into these disorders but also raised mechanistic issues pertaining to the search for the ‘‘principal offender’’ protein in each disease. The pathological hallmarks of AD are neurofibrillary tangles which consist of microtubule-associated protein tau, and senile plaques (SPs) that are composed of amyloid beta peptide (Ah), respectively. Lewy bodies and Lewy neurites, the characteristic lesions in Parkinson’s disease (PD) composed of a-synuclein (a-syn) filaments, also commonly to occur in AD. As frequent and extensive overlap among pathologies with tau, Ah and a-syn is also observed in diverse neurodegenerative disorders, it is difficult to identify the protein that is the most responsible for the neuropathology in each illness. To address this concern, a great number of transgenic (Tg) mice that over-express one of these proteins/peptides have been generated, and have been demonstrated to show loss of normal functions and gain of neurotoxicity of these molecules with progression of fibrillary pathologies. Moreover, generation of double Tg mice that express two of the above-mentioned molecules do develop enhanced fibrillary lesions relative to single Tg mice, indicating that synergic effects of two or more molecules can greatly contribute to the initiation and promotion of neurodegenerative pathologies. D 2003 Elsevier B.V. All rights reserved. Keywords: Alzheimer’s disease; Parkinson’s disease, Amyloid beta peptide; Tau protein, Alpha-synuclein
1. Introduction Diverse neurodegenerative disorders, including Alzheimer’s disease (AD) and Parkinson’s disease (PD), among presenile and senile patients are neuropathologically * Corresponding author. Laboratory for Proteolytic Neuroscience, Riken Brain Science Institute, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan. Tel.: +81-48-462-1111x7613; fax: +81-48-467-9716. E-mail address: [email protected] (M. Higuchi). 0531-5131/ D 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0531-5131(03)01567-X
70
M. Higuchi et al. / International Congress Series 1260 (2004) 69–75
characterized by the accumulation of filamentous protein aggregates in the CNS. Extracellular senile plaques (SPs) composed of amyloid h peptides (Ah) and intracellular neurofibrillary tangles (NFTs) constituted by microtubule-associated protein tau are both neuropathological hallmarks in AD [1]. In PD and dementia with Lewy bodies (DLB), a-synuclein (a-syn) filaments accumulate and form Lewy bodies in neuronal somas and Lewy neurites in neuronal processes [2]. Compelling evidence shows that these proteinous aggregates not only are hallmark lesions to define neuropathological diagnosis of these diseases but also play primary roles in the molecular processes of neurodegeneration. Indeed, the spatial extent of these fibrillary lesions in the brain is closely associated with disease progression and the extent of neuronal loss [3,4]. The most significant findings supporting the roles of these proteins in neurodegeneration stem from the discoveries of mutations on the genes of these molecules that cause familial forms of neurodegenerative diseases [1,5,6], indicating that abnormalities of these molecules alone can give rise to progressive degeneration of neurons and glia in the human CNS. Elucidating the molecular mechanisms of neuronal injuries induced by the accumulations of Ah, tau and a-syn, and clarifying the effects of aging on the properties of these molecules should provide new insights into the exploitation of therapeutic approaches to a number of neurodegenerative disorders. This short review presents plausible explanations for the mechanistic roles of these pathogenic proteins in the molecular processes of neurodegenerative diseases and discusses probable synergic effects among these molecules that create the characteristic neuropathology of each illness. 2. Loss of normal functions and gain of neurotoxicity of AB, tau and A-syn by the formation of pathological filaments in neurodegenerative processes Of these three amyloidogenic molecules, only the normal functions of tau are well understood. Subcellular localization of tau to axons is a common finding among diverse species, and it contributes to the organization of axonal microtubules (MTs) by promoting the polymerization of tubulins and stabilizing the assembled MTs [7]. This dynamic interaction between tau and MTs is regulated by phosphorylation of tau, as phosphorylated tau has a lower binding affinity to MTs than tau in its non-phosphorylated state [7]. Neuronal tau is highly phosphorylated during development, allowing the MT network to be less stable and more flexible in growing axons. Studies on AD and related neurodegenerative illnesses with enormous tau inclusions have revealed that tau in fibrillary lesions is hyperphosphorylated at multiple amino acid residues, a few of which are uniquely phosphorylated in these disorders [1,7]. Hence, it is likely that hyperphosphorylation and aberrant phosphorylation of tau reduce its ability to bind to MTs, and the decrease of MT-bound tau presumably results in deficits of cytoskeletal organization in axons. The observation of hyperphosphorylated tau in AD also raises an additional speculation that the assembly of tau into cytotoxic filaments is facilitated by its phosphorylation. However, none of in vitro experiments on tau filament assembly so far have successfully shown that the phosphorylation of tau promotes its polymerization into filaments [8]. Therefore, it is reasonable to seek other mechanisms underlying the
M. Higuchi et al. / International Congress Series 1260 (2004) 69–75
71
subcellular dislocation of tau from the axon to the somato-dendritic area and the formation of filamentous aggregates. In fact, recent studies on tau gene mutations, which are causative of a group of familial tauopathies referred to as frontotemporal dementia and parkinsonism linked to chromosome 17 (FTDP-17), have demonstrated that some FTDP17 mutations abolish the abilities of tau to bind to MTs and to promote/stabilize MT assembly and facilitate polymerization of tau into pathological filaments [9,10]. Since the level of tau phosphorylation is not altered by FTDP-17 mutations, these observations imply that molecular processes of tau pathogenesis unrelated to its phosphorylation may also exist. Unlike tau, the physiological functions of a-syn in normal conditions remain to be clarified. However, a-syn may play important roles in the formation of synaptic vesicles from early endosomes [11]. Accordingly, the subcellular dislocation of a-syn from synapses to the somato-dendritic area is likely to disrupt synaptic integrity and accelerates somato-dendritic accumulation of cytotoxic a-syn filaments. Functional alterations of asyn in pathological conditions are extensively analyzed in relation to mutations on the asyn gene that have been discovered in kindreds of familial PD. Several recent studies indicated that the potential functions of a-syn in regulating the formation of synaptic vesicles can be significantly impaired by the presence of pathogenic a-syn mutations [12]. The disturbance of the vesicular formation may diminish neurotransmission and elevate the level of cytosolic neurotransmitters. In the dopaminergic presynaptic terminals, for example, excessive cytosolic dopamine can yield superoxide radicals, resulting in increased levels of oxidative stress (Fig. 1). This could be a plausible explanation for the selective degeneration of nigro-striatal dopaminergic neurons in PD. Oxidative insult may be a consequence of the a-syn dysfunction in the synaptic terminals, while it can be in turn a facilitator of a-syn aggregation in the neuronal soma. The interaction between superoxide and nitric oxide generates nitrating agents, which can promote polymerization
Fig. 1. Molecular processes of tau, a-syn and Ah pathologies in neurons. (A) Dislocations of tau and a-syn to the somato-dendritic area disrupt axonal and synaptic integrities and result in formation of cytotoxic filaments. (B) Increased h-cleavage, abnormal g-cleavage producing more Ah42, or impaired Ah degradation induces accumulation of neurotoxic Ah fibrils. AICD: APP intracellular domain.
72
M. Higuchi et al. / International Congress Series 1260 (2004) 69–75
of a-syn by producing cross-links between tyrosine residues on a-syn [13]. Furthermore, studies on brains of patients with PD and DLB have shown that a-syn forming insoluble fibrils in Lewy bodies and Lewy neurites is selectively nitrated, implying that oxidation and nitration contribute to the abnormalities in a-syn dynamics [14]. Since the participation of a-syn to the formation of somato-dendritic inclusions can cause depletion of the presynaptic a-syn, the acceleration of a-syn polymerization potentially has two-fold impacts on the neurons by the neurotoxicity of a-syn filaments and by the loss of normally functioning a-syn in the nerve terminals. Although roles played by Ah have been implicated in the modulation of synaptic excitability, the normal functions of Ah in neurons, like a-syn, are still unknown. Ah is produced by the cleavage of amyloid precursor protein (APP), which undergoes two distinct post-translational processing pathways mediated by three enzymes: a-, h- and g-secretases [15]. Ah generated by the h- and g-cleavages consists of multiple peptides, the two most common species being 40-amino acid Ah (Ah40) and 42amino acid Ah (Ah42). As with Ah, normal functions of APP and its nonamyloidogenic fragments have not been elucidated, although APP localized to synaptic terminals may regulate neurite extension [16]. The major familial AD (FAD) mutations discovered on the APP, presenilin 1 (PS1) and presenilin 2 (PS2) genes have been indicated to increase the level of total Ah or the ratio of more amyloidogenic Ah42 to Ah40. In addition, PS1 and PS2 are suggested to be catalytic components of gsecretase [17]. Yet, increased production of total Ah and/or Ah42 has not been demonstrated in sporadic AD (SAD) cases, which account for approximately 90% of all AD patients. Hence, an accumulation of Ah in SAD may be explained by declined catabolism of Ah and impaired clearance of Ah. Several candidate proteases that are capable of degrading Ah have been indicated in recent in vitro studies. Among them, a few metallopeptidases including neprilysin and insulin-degrading enzyme have further been demonstrated to proteolyze Ah in vivo [18]. Therefore, the activities of these enzymes are likely to decline with aging and in pathological conditions, leading to an elevated level of Ah. Despite the molecular insights into the neurodegeneration resulting from the formation of proteinous fibrils, the highly frequent overlap among pathologies of Ah, tau and a-syn in AD, PD, DLB and associated disorders hinders the estimation of the neuropathological impacts of each pathogenic molecule [19]. This gives a rationale of analyzing genetically engineered animals such as transgenic (Tg) mice over-expressing human APP, tau or a-syn. Notably, filamentous aggregates similar to the fibrillary lesions in human brains have been observed mostly in the use of transgenes with the FAD, FTDP-17 or familial PD mutations [7,20], while the over-expression of the nonmutant human proteins does not generally lead to fibrillary pathologies. These findings indicate that sporadic and/or late-onset neurodegenerative diseases are the products of longevity among humans, which these experimental animal models cannot accomplish. Thus, it would be difficult to create animal models of sporadic AD, PD, etc., by simply over-expressing wild type (non-mutant) Ah, tau or a-syn unless researchers can accelerate aging of animal brains. Although formation of filamentous aggregates is uncommon in non-mutant Tg mice, degeneration of neurons and glia without, or prior to, the emergence of pathological fibrils has been observed in many of these animal
M. Higuchi et al. / International Congress Series 1260 (2004) 69–75
73
models [21,22]. It is thus likely that the generation of oligomeric/protofibrillar Ah, tau and a-syn is causally related to the loss of normal functions and gains in neuro- and glio-toxicity of these proteins. 3. Initiation and promotion of neurotoxic lesions by crosstalk among pathogenic proteins What the Tg models over-expressing a single molecule have not fully demonstrated is the impact of accumulation of one pathogenic protein on the dynamics of the others. AD brains, including FAD cases with the APP mutations, not only show both Ah and tau pathologies without exception but also frequently develop filamentous a-syn lesions [19]. The formation of tau aggregates in AD appears to be a causal factor of neurodegeneration and not a consequence of neurodegeneration caused by Ah pathology, since tau-positive fibrillary lesions in AD brains are more consistently associated with neuronal loss than amyloid plaques [3]. Another important finding is that the accumulation of abnormal tau fibrils in AD is restricted to neurons, while in other tau filament-positive disorders without Ah pathologies, such as FTDP-17, there is a wide variety of tau inclusions in both neurons and glia [23]. Taken together with the fact that, unlike tau, the expression of APP in the brain is specific to neurons, these findings imply that the specificity of fibrillary pathologies to neurons in AD brains is due to initialization of tau pathogenesis by Ah accumulation in neurons. Although APP Tg mice do not develop filamentous tau lesions, APP/tau double Tg mice do show enhanced NFT-like tau inclusions relative to the tau single Tg mice, providing in vivo evidence for the induction of tau pathology by APP/Ah. Similarly, augmentation of a-syn lesions by an increased level of APP/Ah has also been demonstrated in APP/a-syn double Tg mice [24]. Co-existence of tau and a-syn pathologies is also observed in diverse neurodegenerative diseases including AD, PD, DLB and multiple system atrophy (MSA) [19]. Significantly, filamentous tau lesions emerge in familial cases with a-syn mutations, suggesting induction of tau pathology by a-syn abnormalities [25]. Tau and a-syn synergistically promote polymerization of each other into pathological filaments according to a recent in vitro analysis [26]. Moreover, double Tg mice over-expressing tau and a-syn in glia develop fairly abundant fibrillary lesions resembling glial cytoplasmic inclusions in MSA brains, in comparison to tau and a-syn single Tg mice [26]. This synergism explains why the filamentous aggregates are generated in glia which has a relatively small cytoplasm, but not readily in neurons, as tau and a-syn are distinctly localized in normal neurons and thus have little opportunity to interact. An additional potential mechanism underlying co-induction of APP/Ah, tau and a-syn pathologies in neurons may stem from abnormalities of one molecule interfering with the regular subcellular trafficking and metabolism of the others, since all these molecules are transported along the axon and undergo metabolism in the presynaptic terminal. In AD, for instance, the accumulation of protofibrillar Ah in the presynaptic area is likely to show axono-synaptic toxicity, disrupting the regular trafficking of axonal components including tau. The loss of normally functioning tau in the axon may further decline the axonal transport of a-syn, leading to more severe synaptic dysfunctions. During this vicious cycle, the deficits of axono-
74
M. Higuchi et al. / International Congress Series 1260 (2004) 69–75
synaptic functions progress, and cytotoxic tau and a-syn filaments form in the neuronal soma and dendrites. 4. Conclusion Recently these amyloidogenic molecules, Ah, tau and a-syn, have been the focuses of pathobiological studies on the neurodegenerative disorders, since they leave filamentous signatures on brain sections from autopsied patients and animal models. The pathological alterations of these molecules leading to a loss of normal functions and a gain of neurotoxicity have been pursued in terms of the mutations on their genes that cause the FAD, FTDP-17 and familial PD. The neuropathological significance of these molecules is also appreciated in sporadic neurodegenerative illnesses. However, the mechanistic roles of aging in the initiation and progress of Ah, tau and a-syn pathologies in SAD, PD and associated diseases remain unclear. As the crosstalk among these pathogenic molecules greatly contributes to neurodegenerative processes, interactions between these molecules and other proteins, which show alterations in their activities and/or functions over the course of aging, could potentially promote the pathogenesis of sporadic illnesses. Such proteins can be targeted in generating animal models of sporadic neurodegenerative disorders and in exploiting therapeutic approaches to these illnesses. Acknowledgements The studies summarized here from Center for Neurodegenerative Disease Research, University of Pennsylvania were supported by grants from the National Institute on Aging and the Alzheimer’s Association. We thank Ms. Bonnie Lee La Madeleine for proofreading the text. References [1] M. Higuchi, J.Q. Trojanowski, V.M.-Y. Lee, Tau protein and tauopathy, in: K. Davis, D. Charney, J.T. Coyle, C. Nemeroff (Eds.), Neuropsychopharmacology: The Fifth Generation of Progress, Lippincott Williams & Wilkins, Philadelphia, 2002, pp. 1339 – 1354. [2] J.E. Duda, V.M.-Y. Lee, J.Q. Trojanowski, Neuropathology of synuclein aggregates, J. Neurosci. Res. 61 (2) (2000) 121 – 127. [3] P.V. Arriagada, J.H. Growdon, E.T. Hedley-Whyte, B.T. Hyman, Neurofibrillary tangles but not senile plaques parallel duration and severity of Alzheimer’s disease, Neurology 42 (3 Pt 1) (1992) 631 – 639. [4] H. Braak, K. Del Tredici, U. Rub, et al., Staging of brain pathology related to sporadic Parkinson’s disease, Neurobiol. Aging 24 (2) (2003) 197 – 211. [5] C.L. Lendon, F. Ashall, A.M. Goate, Exploring the etiology of Alzheimer’s disease using molecular genetics, JAMA 277 (10) (1997) 825 – 831. [6] J.E. Galvin, V.M.-Y. Lee, J.Q. Trojanowski, Synucleinopathies: clinical and pathological implications, Arch. Neurol. 58 (2) (2001) 186 – 190. [7] M. Higuchi, V.M.-Y. Lee, J.Q. Trojanowski, Tau and axonopathy in neurodegenerative disorders, Neuromolecular Med. 2 (2) (2002) 131 – 150. [8] M. Goedert, R. Jakes, M.G. Spillantini, et al., Assembly of microtubule-associated protein tau into Alzheimer-like filaments induced by sulphated glycosaminoglycans, Nature 383 (6600) (1996) 550 – 553. [9] M. Hong, V. Zhukareva, V. Vogelsberg-Ragaglia, et al., Mutation-specific functional impairments in distinct tau isoforms of hereditary FTDP-17, Science 282 (5395) (1998) 1914 – 1917.
M. Higuchi et al. / International Congress Series 1260 (2004) 69–75
75
[10] M. Goedert, R. Jakes, R.A. Crowther, Effects of frontotemporal dementia FTDP-17 mutations on heparininduced assembly of tau filaments, FEBS Lett. 450 (3) (1999) 306 – 311. [11] D.D. Murphy, S.M. Rueter, J.Q. Trojanowski, V.M.-Y. Lee, Synucleins are developmentally expressed, and alpha-synuclein regulates the size of the presynaptic vesicular pool in primary hippocampal neurons, J. Neurosci. 20 (9) (2000) 3214 – 3220. [12] J. Lotharius, P. Brundin, Impaired dopamine storage resulting from alpha-synuclein mutations may contribute to the pathogenesis of Parkinson’s disease, Hum. Mol. Genet. 11 (20) (2002) 2395 – 2407. [13] J.M. Souza, B.I. Giasson, Q. Chen, et al., Dityrosine cross-linking promotes formation of stable alphasynuclein polymers. Implication of nitrative and oxidative stress in the pathogenesis of neurodegenerative synucleinopathies, J. Biol. Chem. 275 (24) (2000) 18344 – 18349. [14] B.I. Giasson, J.E. Duda, I.V. Murray, et al., Oxidative damage linked to neurodegeneration by selective alpha-synuclein nitration in synucleinopathy lesions, Science 290 (5493) (2000) 985 – 989. [15] D.J. Selkoe, The cell biology of beta-amyloid precursor protein and presenilin in Alzheimer’s disease, Trends Cell Biol. 8 (11) (1998) 447 – 453. [16] U. Muller, N. Cristina, Z.W. Li, et al., Behavioral and anatomical deficits in mice homozygous for a modified beta-amyloid precursor protein gene, Cell 79 (5) (1994) 755 – 765. [17] Y.M. Li, M. Xu, M.T. Lai, et al., Photoactivated gamma-secretase inhibitors directed to the active site covalently label presenilin 1, Nature 405 (6787) (2000) 689 – 694. [18] T. Saito, Y. Takaki, N. Iwata, et al., Alzheimer’s disease, neuropeptides, neuropeptidase, and amyloid-{beta} peptide metabolism, Sci. Aging Knowl. Environ. 2003 (3) (2003) PE1. [19] J.Q. Trojanowski, Tauists, baptists, syners, apostates, and new data, Ann. Neurol. 52 (3) (2002) 263 – 265. [20] P.C. Wong, H. Cai, D.R. Borchelt, D.L. Price, Genetically engineered mouse models of neurodegenerative diseases, Nat. Neurosci. 5 (7) (2002) 633 – 639. [21] M. Higuchi, T. Ishihara, B. Zhang, et al., Transgenic mouse model of tauopathies with glial pathology and nervous system degeneration, Neuron 35 (3) (2002) 433 – 446. [22] A.Y. Hsia, E. Masliah, L. McConlogue, et al., Plaque-independent disruption of neural circuits in Alzheimer’s disease mouse models, Proc. Natl. Acad. Sci. U. S. A. 96 (6) (1999) 3228 – 3233. [23] D.W. Dickson, M.B. Feany, S.H. Yen, et al., Cytoskeletal pathology in non-Alzheimer degenerative dementia: new lesions in diffuse Lewy body disease, Pick’s disease, and corticobasal degeneration, J. Neural Transm., Suppl. 47 (1996) 31 – 46. [24] E. Masliah, E. Rockenstein, I. Veinbergs, et al., Beta-amyloid peptides enhance alpha-synuclein accumulation and neuronal deficits in a transgenic mouse model linking Alzheimer’s disease and Parkinson’s disease, Proc. Natl. Acad. Sci. U. S. A. 98 (21) (2001) 12245 – 12250. [25] J.E. Duda, B.I. Giasson, M.E. Mabon, et al., Concurrence of alpha-synuclein and tau brain pathology in the Contursi kindred, Acta Neuropathol. (Berl.) 104 (1) (2002) 7 – 11. [26] B.I. Giasson, M.S. Forman, M. Higuchi, et al., Initiation and synergistic fibrillization of tau and alphasynuclein, Science 300 (5619) (2003) 636 – 640.
www.ics-elsevier.com
Pathobiological features in neurodegenerative diseases: an overview Makoto Higuchi a,c,*, John Q. Trojanowski b, Virginia M.-Y. Lee a a
Center for Neurodegenerative Disease Research, University of Pennsylvania, Philadelphia, PA 19104, USA b Institute on Aging, University of Pennsylvania, Philadelphia, PA 19104, USA c Laboratory for Proteolytic Neuroscience, Riken Brain Science Institute, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan Received 20 July 2003; received in revised form 29 July 2003; accepted 4 September 2003
Abstract. Discovery and characterization of fibrillary aggregates composed of specific neuronal/glial proteins in Alzheimer’s disease (AD) and other neurodegenerative diseases have not only provided molecular insights into these disorders but also raised mechanistic issues pertaining to the search for the ‘‘principal offender’’ protein in each disease. The pathological hallmarks of AD are neurofibrillary tangles which consist of microtubule-associated protein tau, and senile plaques (SPs) that are composed of amyloid beta peptide (Ah), respectively. Lewy bodies and Lewy neurites, the characteristic lesions in Parkinson’s disease (PD) composed of a-synuclein (a-syn) filaments, also commonly to occur in AD. As frequent and extensive overlap among pathologies with tau, Ah and a-syn is also observed in diverse neurodegenerative disorders, it is difficult to identify the protein that is the most responsible for the neuropathology in each illness. To address this concern, a great number of transgenic (Tg) mice that over-express one of these proteins/peptides have been generated, and have been demonstrated to show loss of normal functions and gain of neurotoxicity of these molecules with progression of fibrillary pathologies. Moreover, generation of double Tg mice that express two of the above-mentioned molecules do develop enhanced fibrillary lesions relative to single Tg mice, indicating that synergic effects of two or more molecules can greatly contribute to the initiation and promotion of neurodegenerative pathologies. D 2003 Elsevier B.V. All rights reserved. Keywords: Alzheimer’s disease; Parkinson’s disease, Amyloid beta peptide; Tau protein, Alpha-synuclein
1. Introduction Diverse neurodegenerative disorders, including Alzheimer’s disease (AD) and Parkinson’s disease (PD), among presenile and senile patients are neuropathologically * Corresponding author. Laboratory for Proteolytic Neuroscience, Riken Brain Science Institute, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan. Tel.: +81-48-462-1111x7613; fax: +81-48-467-9716. E-mail address: [email protected] (M. Higuchi). 0531-5131/ D 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0531-5131(03)01567-X
70
M. Higuchi et al. / International Congress Series 1260 (2004) 69–75
characterized by the accumulation of filamentous protein aggregates in the CNS. Extracellular senile plaques (SPs) composed of amyloid h peptides (Ah) and intracellular neurofibrillary tangles (NFTs) constituted by microtubule-associated protein tau are both neuropathological hallmarks in AD [1]. In PD and dementia with Lewy bodies (DLB), a-synuclein (a-syn) filaments accumulate and form Lewy bodies in neuronal somas and Lewy neurites in neuronal processes [2]. Compelling evidence shows that these proteinous aggregates not only are hallmark lesions to define neuropathological diagnosis of these diseases but also play primary roles in the molecular processes of neurodegeneration. Indeed, the spatial extent of these fibrillary lesions in the brain is closely associated with disease progression and the extent of neuronal loss [3,4]. The most significant findings supporting the roles of these proteins in neurodegeneration stem from the discoveries of mutations on the genes of these molecules that cause familial forms of neurodegenerative diseases [1,5,6], indicating that abnormalities of these molecules alone can give rise to progressive degeneration of neurons and glia in the human CNS. Elucidating the molecular mechanisms of neuronal injuries induced by the accumulations of Ah, tau and a-syn, and clarifying the effects of aging on the properties of these molecules should provide new insights into the exploitation of therapeutic approaches to a number of neurodegenerative disorders. This short review presents plausible explanations for the mechanistic roles of these pathogenic proteins in the molecular processes of neurodegenerative diseases and discusses probable synergic effects among these molecules that create the characteristic neuropathology of each illness. 2. Loss of normal functions and gain of neurotoxicity of AB, tau and A-syn by the formation of pathological filaments in neurodegenerative processes Of these three amyloidogenic molecules, only the normal functions of tau are well understood. Subcellular localization of tau to axons is a common finding among diverse species, and it contributes to the organization of axonal microtubules (MTs) by promoting the polymerization of tubulins and stabilizing the assembled MTs [7]. This dynamic interaction between tau and MTs is regulated by phosphorylation of tau, as phosphorylated tau has a lower binding affinity to MTs than tau in its non-phosphorylated state [7]. Neuronal tau is highly phosphorylated during development, allowing the MT network to be less stable and more flexible in growing axons. Studies on AD and related neurodegenerative illnesses with enormous tau inclusions have revealed that tau in fibrillary lesions is hyperphosphorylated at multiple amino acid residues, a few of which are uniquely phosphorylated in these disorders [1,7]. Hence, it is likely that hyperphosphorylation and aberrant phosphorylation of tau reduce its ability to bind to MTs, and the decrease of MT-bound tau presumably results in deficits of cytoskeletal organization in axons. The observation of hyperphosphorylated tau in AD also raises an additional speculation that the assembly of tau into cytotoxic filaments is facilitated by its phosphorylation. However, none of in vitro experiments on tau filament assembly so far have successfully shown that the phosphorylation of tau promotes its polymerization into filaments [8]. Therefore, it is reasonable to seek other mechanisms underlying the
M. Higuchi et al. / International Congress Series 1260 (2004) 69–75
71
subcellular dislocation of tau from the axon to the somato-dendritic area and the formation of filamentous aggregates. In fact, recent studies on tau gene mutations, which are causative of a group of familial tauopathies referred to as frontotemporal dementia and parkinsonism linked to chromosome 17 (FTDP-17), have demonstrated that some FTDP17 mutations abolish the abilities of tau to bind to MTs and to promote/stabilize MT assembly and facilitate polymerization of tau into pathological filaments [9,10]. Since the level of tau phosphorylation is not altered by FTDP-17 mutations, these observations imply that molecular processes of tau pathogenesis unrelated to its phosphorylation may also exist. Unlike tau, the physiological functions of a-syn in normal conditions remain to be clarified. However, a-syn may play important roles in the formation of synaptic vesicles from early endosomes [11]. Accordingly, the subcellular dislocation of a-syn from synapses to the somato-dendritic area is likely to disrupt synaptic integrity and accelerates somato-dendritic accumulation of cytotoxic a-syn filaments. Functional alterations of asyn in pathological conditions are extensively analyzed in relation to mutations on the asyn gene that have been discovered in kindreds of familial PD. Several recent studies indicated that the potential functions of a-syn in regulating the formation of synaptic vesicles can be significantly impaired by the presence of pathogenic a-syn mutations [12]. The disturbance of the vesicular formation may diminish neurotransmission and elevate the level of cytosolic neurotransmitters. In the dopaminergic presynaptic terminals, for example, excessive cytosolic dopamine can yield superoxide radicals, resulting in increased levels of oxidative stress (Fig. 1). This could be a plausible explanation for the selective degeneration of nigro-striatal dopaminergic neurons in PD. Oxidative insult may be a consequence of the a-syn dysfunction in the synaptic terminals, while it can be in turn a facilitator of a-syn aggregation in the neuronal soma. The interaction between superoxide and nitric oxide generates nitrating agents, which can promote polymerization
Fig. 1. Molecular processes of tau, a-syn and Ah pathologies in neurons. (A) Dislocations of tau and a-syn to the somato-dendritic area disrupt axonal and synaptic integrities and result in formation of cytotoxic filaments. (B) Increased h-cleavage, abnormal g-cleavage producing more Ah42, or impaired Ah degradation induces accumulation of neurotoxic Ah fibrils. AICD: APP intracellular domain.
72
M. Higuchi et al. / International Congress Series 1260 (2004) 69–75
of a-syn by producing cross-links between tyrosine residues on a-syn [13]. Furthermore, studies on brains of patients with PD and DLB have shown that a-syn forming insoluble fibrils in Lewy bodies and Lewy neurites is selectively nitrated, implying that oxidation and nitration contribute to the abnormalities in a-syn dynamics [14]. Since the participation of a-syn to the formation of somato-dendritic inclusions can cause depletion of the presynaptic a-syn, the acceleration of a-syn polymerization potentially has two-fold impacts on the neurons by the neurotoxicity of a-syn filaments and by the loss of normally functioning a-syn in the nerve terminals. Although roles played by Ah have been implicated in the modulation of synaptic excitability, the normal functions of Ah in neurons, like a-syn, are still unknown. Ah is produced by the cleavage of amyloid precursor protein (APP), which undergoes two distinct post-translational processing pathways mediated by three enzymes: a-, h- and g-secretases [15]. Ah generated by the h- and g-cleavages consists of multiple peptides, the two most common species being 40-amino acid Ah (Ah40) and 42amino acid Ah (Ah42). As with Ah, normal functions of APP and its nonamyloidogenic fragments have not been elucidated, although APP localized to synaptic terminals may regulate neurite extension [16]. The major familial AD (FAD) mutations discovered on the APP, presenilin 1 (PS1) and presenilin 2 (PS2) genes have been indicated to increase the level of total Ah or the ratio of more amyloidogenic Ah42 to Ah40. In addition, PS1 and PS2 are suggested to be catalytic components of gsecretase [17]. Yet, increased production of total Ah and/or Ah42 has not been demonstrated in sporadic AD (SAD) cases, which account for approximately 90% of all AD patients. Hence, an accumulation of Ah in SAD may be explained by declined catabolism of Ah and impaired clearance of Ah. Several candidate proteases that are capable of degrading Ah have been indicated in recent in vitro studies. Among them, a few metallopeptidases including neprilysin and insulin-degrading enzyme have further been demonstrated to proteolyze Ah in vivo [18]. Therefore, the activities of these enzymes are likely to decline with aging and in pathological conditions, leading to an elevated level of Ah. Despite the molecular insights into the neurodegeneration resulting from the formation of proteinous fibrils, the highly frequent overlap among pathologies of Ah, tau and a-syn in AD, PD, DLB and associated disorders hinders the estimation of the neuropathological impacts of each pathogenic molecule [19]. This gives a rationale of analyzing genetically engineered animals such as transgenic (Tg) mice over-expressing human APP, tau or a-syn. Notably, filamentous aggregates similar to the fibrillary lesions in human brains have been observed mostly in the use of transgenes with the FAD, FTDP-17 or familial PD mutations [7,20], while the over-expression of the nonmutant human proteins does not generally lead to fibrillary pathologies. These findings indicate that sporadic and/or late-onset neurodegenerative diseases are the products of longevity among humans, which these experimental animal models cannot accomplish. Thus, it would be difficult to create animal models of sporadic AD, PD, etc., by simply over-expressing wild type (non-mutant) Ah, tau or a-syn unless researchers can accelerate aging of animal brains. Although formation of filamentous aggregates is uncommon in non-mutant Tg mice, degeneration of neurons and glia without, or prior to, the emergence of pathological fibrils has been observed in many of these animal
M. Higuchi et al. / International Congress Series 1260 (2004) 69–75
73
models [21,22]. It is thus likely that the generation of oligomeric/protofibrillar Ah, tau and a-syn is causally related to the loss of normal functions and gains in neuro- and glio-toxicity of these proteins. 3. Initiation and promotion of neurotoxic lesions by crosstalk among pathogenic proteins What the Tg models over-expressing a single molecule have not fully demonstrated is the impact of accumulation of one pathogenic protein on the dynamics of the others. AD brains, including FAD cases with the APP mutations, not only show both Ah and tau pathologies without exception but also frequently develop filamentous a-syn lesions [19]. The formation of tau aggregates in AD appears to be a causal factor of neurodegeneration and not a consequence of neurodegeneration caused by Ah pathology, since tau-positive fibrillary lesions in AD brains are more consistently associated with neuronal loss than amyloid plaques [3]. Another important finding is that the accumulation of abnormal tau fibrils in AD is restricted to neurons, while in other tau filament-positive disorders without Ah pathologies, such as FTDP-17, there is a wide variety of tau inclusions in both neurons and glia [23]. Taken together with the fact that, unlike tau, the expression of APP in the brain is specific to neurons, these findings imply that the specificity of fibrillary pathologies to neurons in AD brains is due to initialization of tau pathogenesis by Ah accumulation in neurons. Although APP Tg mice do not develop filamentous tau lesions, APP/tau double Tg mice do show enhanced NFT-like tau inclusions relative to the tau single Tg mice, providing in vivo evidence for the induction of tau pathology by APP/Ah. Similarly, augmentation of a-syn lesions by an increased level of APP/Ah has also been demonstrated in APP/a-syn double Tg mice [24]. Co-existence of tau and a-syn pathologies is also observed in diverse neurodegenerative diseases including AD, PD, DLB and multiple system atrophy (MSA) [19]. Significantly, filamentous tau lesions emerge in familial cases with a-syn mutations, suggesting induction of tau pathology by a-syn abnormalities [25]. Tau and a-syn synergistically promote polymerization of each other into pathological filaments according to a recent in vitro analysis [26]. Moreover, double Tg mice over-expressing tau and a-syn in glia develop fairly abundant fibrillary lesions resembling glial cytoplasmic inclusions in MSA brains, in comparison to tau and a-syn single Tg mice [26]. This synergism explains why the filamentous aggregates are generated in glia which has a relatively small cytoplasm, but not readily in neurons, as tau and a-syn are distinctly localized in normal neurons and thus have little opportunity to interact. An additional potential mechanism underlying co-induction of APP/Ah, tau and a-syn pathologies in neurons may stem from abnormalities of one molecule interfering with the regular subcellular trafficking and metabolism of the others, since all these molecules are transported along the axon and undergo metabolism in the presynaptic terminal. In AD, for instance, the accumulation of protofibrillar Ah in the presynaptic area is likely to show axono-synaptic toxicity, disrupting the regular trafficking of axonal components including tau. The loss of normally functioning tau in the axon may further decline the axonal transport of a-syn, leading to more severe synaptic dysfunctions. During this vicious cycle, the deficits of axono-
74
M. Higuchi et al. / International Congress Series 1260 (2004) 69–75
synaptic functions progress, and cytotoxic tau and a-syn filaments form in the neuronal soma and dendrites. 4. Conclusion Recently these amyloidogenic molecules, Ah, tau and a-syn, have been the focuses of pathobiological studies on the neurodegenerative disorders, since they leave filamentous signatures on brain sections from autopsied patients and animal models. The pathological alterations of these molecules leading to a loss of normal functions and a gain of neurotoxicity have been pursued in terms of the mutations on their genes that cause the FAD, FTDP-17 and familial PD. The neuropathological significance of these molecules is also appreciated in sporadic neurodegenerative illnesses. However, the mechanistic roles of aging in the initiation and progress of Ah, tau and a-syn pathologies in SAD, PD and associated diseases remain unclear. As the crosstalk among these pathogenic molecules greatly contributes to neurodegenerative processes, interactions between these molecules and other proteins, which show alterations in their activities and/or functions over the course of aging, could potentially promote the pathogenesis of sporadic illnesses. Such proteins can be targeted in generating animal models of sporadic neurodegenerative disorders and in exploiting therapeutic approaches to these illnesses. Acknowledgements The studies summarized here from Center for Neurodegenerative Disease Research, University of Pennsylvania were supported by grants from the National Institute on Aging and the Alzheimer’s Association. We thank Ms. Bonnie Lee La Madeleine for proofreading the text. References [1] M. Higuchi, J.Q. Trojanowski, V.M.-Y. Lee, Tau protein and tauopathy, in: K. Davis, D. Charney, J.T. Coyle, C. Nemeroff (Eds.), Neuropsychopharmacology: The Fifth Generation of Progress, Lippincott Williams & Wilkins, Philadelphia, 2002, pp. 1339 – 1354. [2] J.E. Duda, V.M.-Y. Lee, J.Q. Trojanowski, Neuropathology of synuclein aggregates, J. Neurosci. Res. 61 (2) (2000) 121 – 127. [3] P.V. Arriagada, J.H. Growdon, E.T. Hedley-Whyte, B.T. Hyman, Neurofibrillary tangles but not senile plaques parallel duration and severity of Alzheimer’s disease, Neurology 42 (3 Pt 1) (1992) 631 – 639. [4] H. Braak, K. Del Tredici, U. Rub, et al., Staging of brain pathology related to sporadic Parkinson’s disease, Neurobiol. Aging 24 (2) (2003) 197 – 211. [5] C.L. Lendon, F. Ashall, A.M. Goate, Exploring the etiology of Alzheimer’s disease using molecular genetics, JAMA 277 (10) (1997) 825 – 831. [6] J.E. Galvin, V.M.-Y. Lee, J.Q. Trojanowski, Synucleinopathies: clinical and pathological implications, Arch. Neurol. 58 (2) (2001) 186 – 190. [7] M. Higuchi, V.M.-Y. Lee, J.Q. Trojanowski, Tau and axonopathy in neurodegenerative disorders, Neuromolecular Med. 2 (2) (2002) 131 – 150. [8] M. Goedert, R. Jakes, M.G. Spillantini, et al., Assembly of microtubule-associated protein tau into Alzheimer-like filaments induced by sulphated glycosaminoglycans, Nature 383 (6600) (1996) 550 – 553. [9] M. Hong, V. Zhukareva, V. Vogelsberg-Ragaglia, et al., Mutation-specific functional impairments in distinct tau isoforms of hereditary FTDP-17, Science 282 (5395) (1998) 1914 – 1917.
M. Higuchi et al. / International Congress Series 1260 (2004) 69–75
75
[10] M. Goedert, R. Jakes, R.A. Crowther, Effects of frontotemporal dementia FTDP-17 mutations on heparininduced assembly of tau filaments, FEBS Lett. 450 (3) (1999) 306 – 311. [11] D.D. Murphy, S.M. Rueter, J.Q. Trojanowski, V.M.-Y. Lee, Synucleins are developmentally expressed, and alpha-synuclein regulates the size of the presynaptic vesicular pool in primary hippocampal neurons, J. Neurosci. 20 (9) (2000) 3214 – 3220. [12] J. Lotharius, P. Brundin, Impaired dopamine storage resulting from alpha-synuclein mutations may contribute to the pathogenesis of Parkinson’s disease, Hum. Mol. Genet. 11 (20) (2002) 2395 – 2407. [13] J.M. Souza, B.I. Giasson, Q. Chen, et al., Dityrosine cross-linking promotes formation of stable alphasynuclein polymers. Implication of nitrative and oxidative stress in the pathogenesis of neurodegenerative synucleinopathies, J. Biol. Chem. 275 (24) (2000) 18344 – 18349. [14] B.I. Giasson, J.E. Duda, I.V. Murray, et al., Oxidative damage linked to neurodegeneration by selective alpha-synuclein nitration in synucleinopathy lesions, Science 290 (5493) (2000) 985 – 989. [15] D.J. Selkoe, The cell biology of beta-amyloid precursor protein and presenilin in Alzheimer’s disease, Trends Cell Biol. 8 (11) (1998) 447 – 453. [16] U. Muller, N. Cristina, Z.W. Li, et al., Behavioral and anatomical deficits in mice homozygous for a modified beta-amyloid precursor protein gene, Cell 79 (5) (1994) 755 – 765. [17] Y.M. Li, M. Xu, M.T. Lai, et al., Photoactivated gamma-secretase inhibitors directed to the active site covalently label presenilin 1, Nature 405 (6787) (2000) 689 – 694. [18] T. Saito, Y. Takaki, N. Iwata, et al., Alzheimer’s disease, neuropeptides, neuropeptidase, and amyloid-{beta} peptide metabolism, Sci. Aging Knowl. Environ. 2003 (3) (2003) PE1. [19] J.Q. Trojanowski, Tauists, baptists, syners, apostates, and new data, Ann. Neurol. 52 (3) (2002) 263 – 265. [20] P.C. Wong, H. Cai, D.R. Borchelt, D.L. Price, Genetically engineered mouse models of neurodegenerative diseases, Nat. Neurosci. 5 (7) (2002) 633 – 639. [21] M. Higuchi, T. Ishihara, B. Zhang, et al., Transgenic mouse model of tauopathies with glial pathology and nervous system degeneration, Neuron 35 (3) (2002) 433 – 446. [22] A.Y. Hsia, E. Masliah, L. McConlogue, et al., Plaque-independent disruption of neural circuits in Alzheimer’s disease mouse models, Proc. Natl. Acad. Sci. U. S. A. 96 (6) (1999) 3228 – 3233. [23] D.W. Dickson, M.B. Feany, S.H. Yen, et al., Cytoskeletal pathology in non-Alzheimer degenerative dementia: new lesions in diffuse Lewy body disease, Pick’s disease, and corticobasal degeneration, J. Neural Transm., Suppl. 47 (1996) 31 – 46. [24] E. Masliah, E. Rockenstein, I. Veinbergs, et al., Beta-amyloid peptides enhance alpha-synuclein accumulation and neuronal deficits in a transgenic mouse model linking Alzheimer’s disease and Parkinson’s disease, Proc. Natl. Acad. Sci. U. S. A. 98 (21) (2001) 12245 – 12250. [25] J.E. Duda, B.I. Giasson, M.E. Mabon, et al., Concurrence of alpha-synuclein and tau brain pathology in the Contursi kindred, Acta Neuropathol. (Berl.) 104 (1) (2002) 7 – 11. [26] B.I. Giasson, M.S. Forman, M. Higuchi, et al., Initiation and synergistic fibrillization of tau and alphasynuclein, Science 300 (5619) (2003) 636 – 640.