Axonal transport of human α-synuclein slows with aging but is not affected by familial Parkinson's disease-linked mutations

Antiapoptotic property of human α-synuclein in neuronal cell lines is associated with the inhibition of caspase-3 but not caspase-9 activity

Abnormalities of alpha-synuclein (alpha-Syn) are mechanistically linked to Parkinson's disease (PD) and other alpha-synucleinopathies. To gain additional insights into the relationships between alpha-Syn expression and cell death, we examined the effects of expressing human alpha-Syn (Hualpha-Syn) variants on the cellular vulnerability to apoptotic stimuli. We show that the expression of wild-type (WT) and A30P mutant, but not A53T mutant, Hualpha-Syn leads to the protection of neuronal cell lines from apoptosis but not necrosis. Significantly, Hualpha-Syn did not protect non-neuronal cell lines from apoptosis. We also show that A53T mutant is a loss of function in regards to the antiapoptotic property since the expression of WT Hualpha-Syn with an excess of A53T mutant Hualpha-Syn leads to protection of the cells from apoptosis. The antiapoptotic property is specific to human alpha-Syn as neither beta-Syn nor mouse alpha-Syn protected cells from apoptosis, and the carboxy-terminal 20 amino acids are required for the antiapoptotic property. Analyses of capase-3 and caspase-9 activation reveal that the antiapoptotic property of Hualpha-Syn in neuronal cell lines is associated with the attenuation of caspase-3 activity without affecting the caspase-9 activity or the levels of cleaved, active caspase-3. We conclude that Hualpha-Syn modulates the activity of cleaved caspase-3 product in neuronal cell lines.

Aggregation promoting C-terminal truncation of alpha-synuclein is a normal cellular process and is enhanced by the familial Parkinson's disease-linked mutations

Abnormal biology of alpha-synuclein (alpha-Syn) is directly implicated in the pathogenesis of Parkinson's disease and other alpha-synucleinopathies. Herein, we demonstrate that C-terminally truncated alpha-Syn (alpha-SynDeltaC), enriched in the pathological alpha-Syn aggregates, is normally generated from full-length alpha-Syn independent of alpha-Syn aggregation in brains and in cultured cells. The accumulation of alpha-SynDeltaC is enhanced in neuronal cells as compared with nonneuronal cells. Significantly, the expression of familial Parkinson's disease-linked mutant alpha-Syn is associated with the enhanced cellular accumulation of alpha-SynDeltaC. Moreover, substoichiometric amounts of alpha-SynDeltaC enhance the in vitro aggregation of the more abundant full-length alpha-Syn. Finally, cases of alpha-synucleinopathy exhibit increases in the total soluble alpha-Syn and a higher proportion of soluble alpha-SynDeltaC, a condition favoring the aggregation of alpha-Syn. Collectively, our results indicate that the biology behind the generation and accumulation of alpha-SynDeltaC is likely to have relevance for the initiation and the progression of alpha-Syn aggregation in vivo.

Axonal transport defects: a common theme in neurodegenerative diseases

A core pathology central to most neurodegenerative diseases is the misfolding, fibrillization and aggregation of disease proteins to form the hallmark lesions of specific disorders. The mechanisms underlying these brain-specific neurodegenerative amyloidoses are the focus of intense investigation and defective axonal transport has been hypothesized to play a mechanistic role in several neurodegenerative disorders; however, this hypothesis has not been extensively examined. Discoveries of mutations in human genes encoding motor proteins responsible for axonal transport do provide direct evidence for the involvement of axonal transport in neurodegenerative diseases, and this evidence is supported by studies of animal models of neurodegeneration. In this review, we summarize recent findings related to axonal transport and neurodegeneration. Focusing on specific neurodegenerative diseases from a neuropathologic perspective, we highlight discoveries of human motor protein mutations in some of these diseases, as well as illustrate new insights from animal models of neurodegenerative disorders. We also review the current understanding of the biology of axonal transport including major recent findings related to slow axonal transport.

Stabilization of alpha-synuclein protein with aging and familial parkinson's disease-linked A53T mutation

We examined the potential relationship between aging and alpha-synuclein (alpha-Syn) metabolism, both of which are implicated in the pathogenesis of Parkinson's disease (PD) and other alpha-synucleinopathies. During aging,alpha-Syn and beta-Syn mRNA expression in brain decreases, but the protein levels are maintained at high levels. Significantly, the alpha-Syn protein level increases with aging in human substantia nigra. Pulse-chase analyses of alpha-Syn half-lives in neurons and neuronal cell lines indicate that, in mature neurons, the expression of alpha-Syn is regulated by the post-translational stabilization of alpha-Syn protein. Moreover, A53T mutant human alpha-Syn exhibits increased stability in neuronal cell lines, leading to higher levels of the mutant protein in cells and transgenic mice. Inhibitor studies suggest that the proteasomal and lysosomal systems may not be responsible for the differential stabilization or metabolism of alpha-Syn protein in neuronal cells. Because increased stabilization of alpha-Syn protein is associated with increased protein levels and accumulation of pathogenic protein modifications, such as oxidative damage, the stabilization of alpha-Syn with aging may be a significant factor in the pathogenesis of alpha-synucleinopathies.

Synucleins and their relationship to Parkinson’s disease

Parkinson's disease (PD) is one of the most common neurodegenerative motor disorders, marked by chronic progressive loss of neurons in the substantia nigra. It has long been believed that PD is caused by environmental factors. The discovery of genetic factors involved in PD has improved the understanding of the pathology of the disease. The first gene found to be mutated in PD encodes for the presynaptic protein alpha-synuclein. alpha-Synuclein is a major component of Lewy bodies and Lewy neurites, which represent the morphological hallmarks of the disease. The mechanisms by which alpha-synuclein is involved in nigral cell death remain poorly understood. Moreover, the factors triggering the formation of alpha-synuclein-positive inclusion bodies remain enigmatic. Indeed, even the normal cellular functions of alpha-synuclein and of the other synucleins (beta-synuclein and gamma-synuclein) are still unknown. Several lines of evidence suggest that they play a role in the regulation of vesicular turnover under normal nonpathological conditions.

Degradative organelles containing mislocalized alpha-and beta-synuclein proliferate in presenilin-1 null neurons

Presenilin-1 null mutation (PS1 -/-) in mice is associated with morphological alterations and defects in cleavage of transmembrane proteins. Here, we demonstrate that PS1 deficiency also leads to the formation of degradative vacuoles and to the aberrant translocation of presynaptic alpha- and beta-synuclein proteins to these organelles in the perikarya of primary neurons, concomitant with significant increases in the levels of both synucleins. Stimulation of autophagy in control neurons produced a similar mislocalization of synucleins as genetic ablation of PS1. These effects were not the result of the loss of PS1 gamma-secretase activity; however, dysregulation of calcium channels in PS1 -/- cells may be involved. Finally, colocalization of alpha-synuclein and degradative organelles was observed in brains from patients with the Lewy body variant of AD. Thus, aberrant accumulation of alpha- and beta-synuclein in degradative organelles are novel features of PS1 -/- neurons, and similar events may promote the formation of alpha-synuclein inclusions associated with neurodegenerative diseases.