Synuclein Deficiency Decreases the Efficiency of Dopamine Uptake by Synaptic Vesicles.

In the present study, we analyzed the uptake of radiolabeled dopamine by intact synaptosomes and purified synaptic vesicles isolated from the dorsal striatum of mice with constitutive inactivation of all three synuclein-coding genes and wild-type mice. Synuclein deficiency substantially compromised the uptake of this neurotransmitter by synaptic vesicles but had no effect on synaptosomal dopamine uptake.

Systematic mutagenesis of α-synuclein reveals distinct sequence requirements for physiological and pathological activities.

α-Synuclein is an abundant presynaptic protein that binds to phospholipids and synaptic vesicles. Physiologically, α-synuclein functions as a SNARE-protein chaperone that promotes SNARE-complex assembly for neurotransmitter release. Pathologically, α-synuclein mutations and α-synuclein overexpression cause Parkinson's disease, and aggregates of α-synuclein are found as Lewy bodies in multiple neurodegenerative disorders ("synucleinopathies"). The relation of the physiological functions to the pathological effects of α-synuclein remains unclear. As an initial avenue of addressing this question, we here systematically examined the effect of α-synuclein mutations on its physiological and pathological activities. We generated 26 α-synuclein mutants spanning the entire molecule, and analyzed them compared with wild-type α-synuclein in seven assays that range from biochemical studies with purified α-synuclein, to analyses of α-synuclein expression in cultured neurons, to examinations of the effects of virally expressed α-synuclein introduced into the mouse substantia nigra by stereotactic injections. We found that both the N-terminal and C-terminal sequences of α-synuclein were required for its physiological function as SNARE-complex chaperone, but that these sequences were not essential for its neuropathological effects. In contrast, point mutations in the central region of α-synuclein, referred to as nonamyloid ß component (residues 61-95), as well as point mutations linked to Parkinson's disease (A30P, E46K, and A53T) increased the neurotoxicity of α-synuclein but did not affect its physiological function in SNARE-complex assembly. Thus, our data show that the physiological function of α-synuclein, although protective of neurodegeneration in some contexts, is fundamentally distinct from its neuropathological effects, thereby dissociating the two activities of α-synuclein.

αßγ-Synuclein triple knockout mice reveal age-dependent neuronal dysfunction.

Synucleins are a vertebrate-specific family of abundant neuronal proteins. They comprise three closely related members, α-, ß-, and γ-synuclein. α-Synuclein has been the focus of intense attention since mutations in it were identified as a cause for familial Parkinson's disease. Despite their disease relevance, the normal physiological function of synucleins has remained elusive. To address this, we generated and characterized αßγ-synuclein knockout mice, which lack all members of this protein family. Deletion of synucleins causes alterations in synaptic structure and transmission, age-dependent neuronal dysfunction, as well as diminished survival. Abrogation of synuclein expression decreased excitatory synapse size by ∼30% both in vivo and in vitro, revealing that synucleins are important determinants of presynaptic terminal size. Young synuclein null mice show improved basic transmission, whereas older mice show a pronounced decrement. The late onset phenotypes in synuclein null mice were not due to a loss of synapses or neurons but rather reflect specific changes in synaptic protein composition and axonal structure. Our results demonstrate that synucleins contribute importantly to the long-term operation of the nervous system and that alterations in their physiological function could contribute to the development of Parkinson's disease.

Combinational losses of synucleins reveal their differential requirements for compensating age-dependent alterations in motor behavior and dopamine metabolism.

Synucleins are involved in multiple steps of the neurotransmitter turnover, but the largely normal synaptic function in young adult animals completely lacking synucleins suggests their roles are dispensable for execution of these processes. Instead, they may be utilized for boosting the efficiency of certain molecular mechanisms in presynaptic terminals, with a deficiency of synuclein proteins sensitizing to or exacerbating synaptic malfunction caused by accumulation of mild alterations, which are commonly associated with aging. Although functional redundancy within the family has been reported, it is unclear whether the remaining synucleins can fully compensate for the deficiency of a lost family member or whether some functions are specific for a particular member. We assessed several structural and functional characteristics of the nigrostriatal system of mice lacking members of the synuclein family in every possible combination and demonstrated that stabilization of the striatal dopamine level depends on the presence of α-synuclein and cannot be compensated by other family members, whereas ß-synuclein is required for efficient maintenance of animal's balance and coordination in old age.

Role of synucleins in Alzheimer's disease.

Alzheimer's disease (AD) and Parkinson's disease (PD) are the most common causes of dementia and movement disorders in the elderly. While progressive accumulation of oligomeric amyloid-beta protein (Abeta) has been identified as one of the central toxic events in AD leading to synaptic dysfunction, accumulation of alpha-synuclein (alpha-syn) resulting in the formation of oligomers has been linked to PD. Most of the studies in AD have been focused on investigating the role of Abeta and Tau; however, recent studies suggest that alpha-syn might also play a role in the pathogenesis of AD. For example, fragments of alpha-syn can associate with amyloid plaques and Abeta promotes the aggregation of alpha-syn in vivo and worsens the deficits in alpha-syn tg mice. Moreover, alpha-syn has also been shown to accumulate in limbic regions in AD, Down's syndrome, and familial AD cases. Abeta and alpha-syn might directly interact under pathological conditions leading to the formation of toxic oligomers and nanopores that increase intracellular calcium. The interactions between Abeta and alpha-syn might also result in oxidative stress, lysosomal leakage, and mitochondrial dysfunction. Thus, better understanding the steps involved in the process of Abeta and alpha-syn aggregation is important in order to develop intervention strategies that might prevent or reverse the accumulation of toxic proteins in AD.