Interactions among alpha-synuclein, dopamine, and biomembranes: some clues for understanding neurodegeneration in Parkinson's disease

Parkinson's disease (PD) is a neurologic disorder resulting from the loss of dopaminergic neurons in the brain. Two lines of evidence suggest that the protein alpha-synuclein plays a role in the pathogenesis of PD: Fibrillar alpha-synuclein is a major component of Lewy bodies in diseased neurons, and two mutations in alpha-synuclein are linked to early-onset disease. Accordingly, the fibrillization of alpha-synuclein is proposed to contribute to neurodegeneration in PD. In this report, we provide evidence that oligomeric intermediates of the alpha-synuclein fibrillization pathway, termed protofibrils, might be neurotoxic. Analyses of protofibrillar alpha-synuclein by atomic force microscopy and electron microscopy indicate that the oligomers consist of spheres, chains, and rings. alpha-Synuclein protofibrils permeabilize synthetic vesicles and form pore-like assemblies on the surface of brain-derived vesicles. Dopamine reacts with alpha-synuclein to form a covalent adduct that slows the conversion of protofibrils to fibrils. This finding suggests that cytosolic dopamine in dopaminergic neurons promotes the accumulation of toxic alpha-synuclein protofibrils, which might explain why these neurons are most vulnerable to degeneration in PD. Finally, we note that aggregation of alpha-synuclein likely occurs via different mechanisms in the cell versus the test tube. For example, the binding of alpha-synuclein to cellular membranes might influence its self-assembly. To address this point, we have developed a yeast model that might enable the selection of random alpha-synuclein mutants with different membrane-binding affinities. These variants might be useful to test whether membrane binding by alpha-synuclein is necessary for neurodegeneration in transgenic animal models of PD.

Filamentous nerve cell inclusions in neurodegenerative diseases: tauopathies and alpha-synucleinopathies

Alzheimer's disease and Parkinson's disease are the most common neurodegenerative diseases. They are characterized by the degeneration of selected populations of nerve cells that develop filamentous inclusions before degeneration. The neuronal inclusions of Alzheimer's disease are made of the microtubule-associated protein tau, in a hyperphosphorylated state. Recent work has shown that the filamentous inclusions of Parkinson's disease are made of the protein alpha-synuclein and that rare, familial forms of Parkinson's disease are caused by missense mutations in the alpha-synuclein gene. Besides Parkinson's disease, the filamentous inclusions of two additional neurodegenerative diseases, namely dementia with Lewy bodies and multiple system atrophy, have also been found to be made of alpha-synuclein. Abundant filamentous tau inclusions are not limited to Alzheimer's disease. They are the defining neuropathological characteristic of frontotemporal dementias such as Pick's disease, and of progressive supranuclear palsy and corticobasal degeneration. The recent discovery of mutations in the tau gene in familial forms of frontotemporal dementia has provided a direct link between tau dysfunction and dementing disease. The new work has established that tauopathies and alpha-synucleinopathies account for most late-onset neurodegenerative diseases in man. The formation of intracellular filamentous inclusions might be the gain of toxic function that leads to the demise of affected brain cells.

The role of cystatin C in cerebral amyloid angiopathy and stroke: cell biology and animal models

A variant of the cysteine protease inhibitor, cystatin C, forms amyloid deposited in the cerebral vasculature of patients with hereditary cerebral hemorrhage with amyloidosis, Icelandic type (HCHWA-I), leading to cerebral hemorrhages early in life. However, cystatin C is also implicated in neuronal degenerative diseases in which it does not form the amyloid protein, such as Alzheimer disease (AD). Accumulating data suggest involvement of cystatin C in the pathogenic processes leading to amyloid deposition in cerebral vasculature and most significantly to cerebral hemorrhage in patients with cerebral amyloid angiopathy (CAA). This review focuses on cell culture and animal models used to study the role of cystatin C in these processes.

Fibrillogenesis of tau: insights from tau missense mutations in FTDP-17

Frontotemporal dementia and Parkinsonism linked to chromosome 17 (FTDP-17) is a neurological disorder associated with tau pathology.Tau deposits in FTDP-17 brains consist of polymerized filaments of hyperphosphorylated tau, the morphology of which is determined by the nature of the tau gene mutation observed in each case. A number of mutations associated with FTDP-17 have been identified in the 5' splice site of exon 10 and in exons 9-13 of the tau gene. The exon 10 5' splice site mutations disrupt alternative splicing and thus alter the ratio of 4R and 3R Tau isoforms. The majority of Tau missense mutations decrease its ability to bind tubulin and promote microtubule assembly. The extent of reduction varies depending on the site and nature of the mutation. Some Tau missense mutations also have a direct effect on the rate and the extent of tau filament formation. In the presence of polymerization-inducing agents such as heparin or arachidonic acid, mutant tau forms polymers more efficiently than wild type tau in vitro. Tau mutations affect polymerization at both nucleation and elongation phases. One mutation (R406W) is also known to alter the susceptibility of tau to phosphorylation. Expression of mutant tau in cultured cells changes the cytoskeletal integrity of CHO and COS-7 cells, but none of the tau transfected cells display tau filament inclusions. These findings suggest involvement of at least two mechanisms in the pathogenesis of FTDP-17.

Polyglutamine and polyalanine expansions in ataxin7 result in different types of aggregation and levels of toxicity

Spinocerebellar ataxia type 7 (SCA7) is caused by expansion of a (CAG)n repeat in the ataxin7 gene, resulting in an abnormally long polyglutamine polyQ tract in the translated protein that aggregates in the form of neuronal intranuclear inclusions. Polyalanine (polyA) stretches, implicated in several genetic disorders, also appear to aggregate. To investigate the role of the aggregates in the pathologies, we compared the effects of ataxin7 containing a polyA (ataxin7 - 90A) or polyQ (ataxin7 - 100Q) expansion in HEK 293 cells and in primary cultures of rat mesencephalon. Both proteins formed nuclear and perinuclear aggregates that contained molecular chaperones and components of the ubiquitin-proteasome system, suggesting that they were abnormally folded. Ataxin-90A aggregates differed morphologically from ataxin7 - 100Q aggregates, consisted of small and amorphous rather than fibrillar inclusions and were more toxic to mesencephalic neurons, suggesting that toxicity was determined by the type of aggregate rather than the cellular misfolding response.

Relationship between microtubule-binding repeats and morphology of neurofibrillary tangle in Alzheimer's disease

OBJECTIVE

The present study was performed to compare the distributions of three-repeat (3R) and four-repeat (4R) neurofibrillary tangles (NFT) with those of pretangles (p-NFT), intracellular NFT (i-NFT), and extracellular NFT (e-NFT) in the hippocampus of Alzheimer's disease brains.

METHODS

NFT labeling was performed using anti-tau antibodies: pSer262 for p-NFT, pSer422 for i-NFT, AT8 for e-NFT, RD3 for 3R, and RD4 for 4R tau, and Gallyas impregnation for the NFT population. RD4- and pSer422-positive NFT were detected predominantly in sectors from CA2 to CA4, while RD3- and pSer262-positive NFT were predominantly present in CA1, the entorhinal cortex, and the subiculum. The tau epitope recognized by pSer262 belongs to 4R tau but it showed a strong correlation with RD3- and AT8-positive NFT.

CONCLUSIONS

Sectors CA2-CA4 showed predominantly 4R-NFT containing the pSer422 epitope. pSer262 may detect the process of transformation from p-NFT to i-NFT, and e-NFT consisted predominantly of 3R tau.

Familial amyloidotic polyneuropathy: protein aggregation in the peripheral nervous system

The molecular pathology underlying transthyretin (TTR)-related amyloidosis is largely unknown. It is possible that a common factor in the amyloidogenesis process exists among the different forms; this common factor can involve changes produced by mutations in the three-dimensional structure of TTR, rendering it prone to deposition as amyloid. This amyloidogenic potential, together with other yet unidentified factors, contribute to amyloid deposition. The factors that trigger fibril formation and/or neurodegeneration in TTR-related amyloidosis present central questions for which there are still no available clues. We recently showed in vitro that TTR fibrils trigger NF-kappaB activation, and subsequent studies identified some inflammatory and apoptotic pathways opening perspectives to understand the neurodegeneration process in familial amyloidotic polyneuropathy (FAP). It is current opinion that the modified TTR represents an amyloidogenic intermediate, which integrates the fibril structure; analyses of FAP fibrils have proved that TTR in the fibrils maintains a beta-conformation and have suggested that the TTR monomer is the building block in fibrils. This concept has been questioned recently by investigators, and only future studies on native and synthetic TTR fibrils using high-resolution structural techniques will further elucidate fibril structure and the aggregation pathway. Modulators responsible for phenotypic diversity can be addressed by mice transgenic for different human TTR mutations. Different lines are now available; incomplete penetrance and environmental influence on the deposition of mutant TTR has been observed. Therefore, these animals constitute important tools to address modulators of phenotypic expression and pathophysiological consequences of amyloid deposition at cellular/molecular levels. They are pivotal for testing potential drugs for TTR amyloidosis as well.

Parkinson's syndrome associated with neurofibrillary degeneration and tau pathologic findings

Several distinct clinical syndromes presenting with parkinsonism have been associated with subcortical neurofibrillary degeneration and the abnormal accumulation of hyperphosphorylated tau protein in the brain. Mutations of tau have been linked with a small number of autosomal dominantly inherited families who present with frontolimbic cognitive deficits, behavioural disorders, and Parkinson's syndrome. Some of the sporadic disorders (progressive supranuclear palsy [PSP] and corticobasal degeneration) have been referred to by molecular pathologists as primary tauopathies, implicating abnormalities of tau in their pathogenesis. We have identified a sporadic parkinsonian syndrome characterised by bradykinesia, a variable response to levodopa, and a mean duration of disease of 9 years, which resembles bodig (Parkinson's-dementia of Guam), and histologically has close similarities with both PSP and postencephalitic parkinsonism. Further characterisation of these cases frequently confused with Parkinson's disease may broaden the clinical spectrum of parkinsonian disorders linked with neurofibrillary tangle formation.

GM3 Ganglioside Linked to Neurofibrillary Pathology in a Transgenic Rat Model for Tauopathy

Glycosphingolipids (GSLs) are amphipathic lipids composed of a sphingoid base and a fatty acyl attached to a saccharide moiety. GSLs play an important role in signal transduction, directing proteins within the membrane, cell recognition, and modulation of cell adhesion. Gangliosides and sulfatides belong to a group of acidic GSLs, and numerous studies report their involvement in neurodevelopment, aging, and neurodegeneration. In this study, we used an approach based on hydrophilic interaction liquid chromatography (HILIC) coupled to high-resolution tandem mass spectrometry (HRMS/MS) to characterize the glycosphingolipid profile in rat brain tissue. Then, we screened characterized lipids aiming to identify changes in glycosphingolipid profiles in the normal aging process and tau pathology. Thorough screening of acidic glycosphingolipids in rat brain tissue revealed 117 ganglioside and 36 sulfatide species. Moreover, we found two ganglioside subclasses that were not previously characterized-GT1b-Ac2 and GQ1b-Ac2. The semi-targeted screening revealed significant changes in the levels of sulfatides and GM1a gangliosides during the aging process. In the transgenic SHR24 rat model for tauopathies, we found elevated levels of GM3 gangliosides which may indicate a higher rate of apoptotic processes.