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| This article will appear in a forthcoming issue of Trends in Molecular Medicine. | |
Abstract
Neurodegenerative diseases of the human brain comprise a variety of disorders that for demographic reasons affect an increasing percentage of the aging population. Alzheimer's disease (AD) and Parkinson's disease (PD) are examples of such late-onset diseases. AD is the most common form of dementia, whereas PD is the most common movement disorder. In both diseases, histopathology reveals abundant protein deposits in selected neuronal populations that form prior to neuronal degeneration.
Proteinaceous Aggregates in AD and PD
Neuropathologically, AD is characterized by extracellular β-amyloid-containing plaques that consist mainly of aggregated Aβ peptide derived by proteolysis of the amyloid precursor protein (APP), and intracellular neurofibrillary tangles that are composed of hyperphosphorylated microtubule-associated protein tau (figure 1) [1]. In the absence of amyloid plaques, they are also abundant in other neurodegenerative diseases, including frontotemporal dementia with Parkinsonism linked to chromosome 17 (FTDP-17). The identification of mutations in the tau gene that are linked to FTDP-17 established that dysfunction of tau alone can lead to neurodegeneration and dementia [2].
Parkinson's disease is marked by fibrillar cytoplasmic inclusions that are abundant in degenerating dopaminergic neurons of the substantia nigra and in several other brain regions (figure 1) [3]. These inclusions are known as Lewy bodies (LBs) and Lewy neurites, and mainly contain ubiquitin and α-synuclein. Under physiological conditions, the latter is localized to nerve terminals, with little staining of somata and dendrites. Rare cases of familial PD have been linked to mutations in the genes encoding α-synuclein (A30P and A53T) or parkin, but the cause of the more commonly encountered sporadic disease remains unknown.
LBs are also abundant in dementia with Lewy bodies (DLB) and the LB variant of AD [4]. The availability of α-synuclein-specific antibodies provided a new tool for the identification of LBs in brains from cases of sporadic and familial AD, Down's syndrome, and the parkinsonism-dementia complex of Guam. In one study, for example, LBs were detected in the amygdala of more than half of all familial AD cases, and some LBs colocalized with tau-positive neurofibrillary tangles [5]. Although it is possible that LBs are a nonspecific outcome of end-stage AD, LBs might reflect the co-occurrence of PD in a subset of AD patients (figure 1). Alternatively, the accumulation of LBs in the AD brain might result from the same pathogenic mechanisms that underlie AD and related tauopathies. Although neurofibrillary tangles and amyloid plaques are the hallmark lesions of AD, unknown genetic or epigenetic factors might predispose neurons to accumulate LBs during the progression of AD in a subset of affected patients [5].
Experimental Animal Models of PD
Over the past two years, several reports have described attempts to mimic aspects of the human LB pathology in transgenic mice and flies. Expression of human α-synuclein carrying the pathogenic mutation A30P (hA30P-SYN) in mice resulted in a diffuse cytosolic accumulation of α-synuclein in many neurons throughout the brain, with no selectivity for the substantia nigra [6]. Another group expressed both human wild-type and A53T-SYN in mice, which also caused an amorphous immunoreactivity in the cytosol, and pronounced ubiquitin staining. In addition, the Campbell-Switzer pyridine silver technique revealed a characteristic PD-like staining of motor neurons. A decline of motor performance was observed, along with axonal degeneration and muscle atrophy [7]. The motor phenotype, however, resembled the one observed in many tau transgenic models [8]. It thus might not represent a specific PD phenotype, but rather be related to high expression levels of the respective transgenes in spinal cord.
By contrast, Masliah et al. reported the presence of intranuclear and cytoplasmic α-synuclein-positive inclusions in wild-type SYN transgenic mice, which was identified by electron microscopy [9]. The inclusions were strongly immunoreactive for human but not mouse a-synuclein. This observation is reminiscent of FTDP-17 human tau transgenic models, in which murine endogenous tau is excluded from tau filaments [10]. In the SYN transgenic mice, accumulation of the transgene product was found in many brain regions and only occasionally in the substantia nigra. Dopaminergic nerve terminals were lost in the basal ganglia - a hallmark feature of PD. In addition, a motor phenotype was reported.
The question arises how accurately this mouse model recapitulates the human pathology. Similarities include the localization of α-synuclein deposits in the neocortex and in dopaminergic neurons of the substantia nigra and their reactivity with antibodies to human α-synuclein or ubiquitin. However, the deposits differ from human LBs by being less circumscribed, being localized to the nucleus, and lacking fibrillar components [9]. By contrast, expression of either wild-type or mutant (A30P and A53T) human SYN in nerve cells of the fruitfly Drosophila melanogaster leads to the formation of filamentous intraneuronal inclusions which resemble LBs. Dopaminergic neurons containing such inclusions underwent an age-dependent degeneration, whereas others appeared to be largely unaffected. In addition, the flies displayed locomotor dysfunction [11]. Compared with transgenic mice, the flies formed filaments within only a few days, suggesting that vertebrates might have evolved mechanisms that impair the assembly of α-synuclein.
Pathologies Combined
Using the power of combined transgenesis in mice, new work by Masliah and colleagues now indicates that the pathology in SYN transgenic mice [9] can be enhanced by the simultaneous expression of a human APP transgene [12]. In their new study, both transgenes were driven by the same promoter and conferred widespread expression of APP and α-synuclein. Double transgenic mice revealed an enhancement of locomotor deficits at six months of age when compared with hemizygous single transgenic mice. However, at 12 months of age, the deficits in the single SYN-transgenic mice were similar to those of the double transgenic mice. Homozygous single-transgenic mice were not included as controls. Motor deficits of double transgenic mice were linked to the presence of α-synuclein, whereas the deficits in spatial learning and memory correlated with hAPP/β-amyloid expression. In SYN/APP mice, an age-dependent loss of cholinergic neurons was observed in the nucleus basalis and the caudoputamen, consistent with the loss of cholinergic neurons in the caudoputamen of AD patients with or without LBs. Between three and 20 months of age, accumulation of human α-synuclein in neocortex was 1.6 times higher in the hSYN/hAPP mice, compared with hSYN mice. For comparison, in DLB, LBs are numerous in cortical brain areas, whereas in PD the LB pathology is mainly restricted to the substantia nigra. The observed increase of α-synuclein deposits in the hSYN/hAPP mice was not very dramatic and varied quite a lot. More remarkably, approximately 15% of the intraneuronal inclusions in the hSYN/hAPP mice were fibrillar, whereas all inclusions in the hSYN mice were amorphous. In vitro experiments reported in the same article showed a dramatic effect of the β-amyloid species Aβ42 on the intracellular accumulation of α-synuclein compared with Aβ40-treated or untreated cultures. Similarly, in a cell-free assay, Aβ42 promoted the oligomerization of a-synuclein, whereas Aβ40 did not.
These findings are supported by related studies in FTDP-17 tau transgenic mice which showed that APP and in particular Aβ42 induced a 5-7 fold increase in tangle formation [13,14]. Similarly, injection of Aβ42 into SYN transgenic mice might also induce fibril formation of α-synuclein, as reported for the hAPP/hSYN double transgenic mice.
Although all these models recapitulate aspects of the human pathology and in particular point to a role for β-amyloid in an extended spectrum of diseases, the mechanism by which β-amyloid exerts its effects is still unclear. In AD with LBs, amyloid might initially render highly vulnerable neurons more susceptible to develop LBs, and subsequently lead to a spreading of the LB pathology to less vulnerable neurons, a model proposed by Delacourte for the spreading of neurofibrillary tangles in AD (figure 2) [15].
Future Directions
A robust mouse model for PD that displays authentic LBs is still not available. Simultaneous overexpression of mutant α-synuclein comprising both A30P and A53T mutations, together with APP, could lead to a more pronounced phenotype. For comparison, work in cell culture has shown that the presence of triple mutations of tau (V337M, P301L, and R406W), but not the single mutations alone, leads to amorphous and fibrillar tau aggregates in transfected cell lines [16]. As shown in rodent animal models, different forms of stress (cold water, starvation or heat shock) induce a pre-tangle tau pathology. Similarly, in PD, oxidative stress or environmental toxins might be pathogenic as most cases of PD are idiopathic.
In rat, chronic administration of rotenone, an inhibitor of the mitochondrial complex I, causes the degeneration of dopaminergic nerve cells in the substantia nigra, resulting in hypokinesia and rigidity. Nerve cell death is preceded by the appearance of LB-like inclusions that are immunoreactive for ubiquitin and α-synuclein [17]. Similar experiments have not been reported for mice. If species differences were the reason for a selective susceptibility of rats, the production of transgenic rats may be the method of choice to circumvent the use of such toxins to generate a PD model.
Nonetheless, the present transgenic mouse models are now available for microarray and proteomic analyses to identify genes and proteins involved in the patho-cascade of PD and AD. This will help in determining whether the two pathologies are only overlapping, or, what seems more likely, synergistic.
Despite the lack of a complete model for PD, transgenic experiments now have shown through combining transgenes that β-synuclein, a non-fibrillogenic homologue of α-synuclein, can inhibit α-synuclein aggregation in vivo [18]. A similar class of endogenous factors might regulate the aggregation state of other molecules involved in neurodegeneration. Furthermore, potential drugs aimed at blocking the accumulation of Aβ, α-synuclein or tau might benefit a broader spectrum of neurodegenerative disorders than previously anticipated [12].
The Significance of Tau and Alpha-Synuclein Inclusions in Neurodegenerative Diseases - reviews recent progress in understanding these diseases, with an emphasis on what is known about the significance of intracellular inclusions. From Current Opinion in Genetics and Development, 2001, 11:3:343-351. Full text available from BioMedNet.
Alpha Synuclein Aggregation: Is It the Toxic Gain of Function Responsible for Neurodegeneration in Parkinson's Disease? - reviews recent data on how α-synuclein aggregation may occur, what cellular events might be responsible, and how this may interfere with normal cellular function. From Mechanisms of Ageing and Development, 2001, 122:14:1499 - 1510. Full text available from BioMedNet.
Engineered Modeling and the Secrets of Parkinson's Disease - reviews the contribution of transgenic and gene-targeting technologies to our current understanding of the pathophysiology, etiology, and pathogenesis of Parkinson's disease. From Trends in Neurosciences, 2001, 2001:24:S49-S55. Full text available from BioMedNet.
Transgenic Mouse Models of Alzheimer's Disease - offers insights into the pathogenesis of the disease and potential therapeutic interventions based on new transgenic mouse models. From Trends in Genetics, 2001, 17:10:S7-S12. Full text available from BioMedNet.
Genes, Models and Alzheimer's Disease - reviews recent advances. From Trends in Genetics, 2001, 17:5:254-261. Full text available from BioMedNet.
Alzheimer Research Forum - provides extensive resources for researchers and physicians as well as general information for the public.
Alzheimer's Disease: Unraveling the Mystery - an online booklet produced by the National Institute on Aging. Provides a general description of the disease, including possible causes and treatments.
Alzheimer's Association - contains extensive general information about the disease as well as information on drug development, drug trials, and articles on causes and treatments of the disease from the Association's bulletin Advances.
Alzheimer Web - an excellent collection of frequently updated AD resources for researchers.
Alzheimer's Disease Review - a free Internet journal from the Sanders-Brown Center on Aging at the University of Kentucky.
Parkinson's Web - offers information for the layperson in such areas as disease diagnosis, coping, support resources, and medical treatment.
National Parkinson Foundation - keeps a good list of current news and events relating to PD.
Neurosciences on the Internet - a comprehensive, searchable index of neuroscience resources.
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