Functional Alterations to the Nigrostriatal System in Mice Lacking All Three Members of the Synuclein Family

Abstract

The synucleins (α, β and γ) are highly homologous proteins thought to play a role in regulating neurotransmission and are found abundantly in presynaptic terminals. To overcome functional overlap between synuclein proteins and to understand their role in presynaptic signalling from mesostriatal dopaminergic neurons, we produced mice lacking all three members of the synuclein family. The effect on the mesostriatal system was assessed in adult (4-14 month old) animals using a combination of behavioural, biochemical, histological and electrochemical techniques. Adult triple synuclein null (TKO) mice displayed no overt phenotype, and no change in the number of midbrain dopaminergic neurons. TKO mice were hyperactive in novel environments and exhibited elevated evoked release of dopamine in the striatum detected with fast-scan cyclic voltammetry. Elevated dopamine release was specific to the dorsal not ventral striatum and was accompanied by a decrease of dopamine tissue content. We confirmed a normal synaptic ultrastructure and a normal abundance of SNARE protein complexes in the dorsal striatum. Treatment of TKO animals with drugs affecting dopamine metabolism revealed normal rate of synthesis, enhanced turnover and reduced presynaptic striatal dopamine stores. Our data uniquely reveal the importance of the synuclein proteins in regulating neurotransmitter release from specific populations of midbrain dopamine neurons through mechanisms which differ from those reported in other neurons. The finding that the complete loss of synucleins leads to changes in dopamine handling by presynaptic terminals specifically in those regions preferentially vulnerable in Parkinson’s disease (PD) may ultimately inform on the selectivity of the disease process.

Impaired neurotransmission caused by overexpression of α-synuclein in nigral dopamine neurons

“…These data are consistent with previous studies showing that the synuclein family of proteins are likely to play a role both in normal physiological neurotransmitter release (11,12,19,20) and in the regulation of transporter function (21,22). In vitro studies have shown that overexpression of α-synuclein inhibits the release of catecholamines from adrenal chromaffin and PC12 cells, probably owing to a reduced pool of readily releasable vesicles (9).…”

Deficits in dopaminergic transmission precede neuron loss and dysfunction in a new Parkinson model

“…Furthermore, this deficit was localized to the dopamine inputs to dorsal striatum. These findings are reminiscent of our previous work on α-and γ-synuclein double-knockout and α-, β-, γ-synuclein triple-knockout mice, in which we showed that synucleins negatively regulate dopamine release in the dorsal but not ventral striatum (16,20).…”

“…(B) Rotarod performance was impaired in 18-but not in 3-mo-old SNCA-OVX mice. Two-way ANOVA of square-root transformed data: main effect of age: F (1,28) = 25.95, ***P < 0.001; main effect of genotype: F (2,27) = 3.49, *P < 0.05; genotype/age interaction: F (2,27) = 6.14, **P < 0.01; separate ANOVAs revealed no main effect of genotype at 3 mo (F < 1, P > 0.05), but a significant main effect of genotype at 18 mo (F (2,15) = 10.04, **P < 0.01) (Tukey post hoc test *P < 0.05, **P < 0.01), n = 3-8), which remained significant when body weight was included as a covariate (ANCOVA: main effect of genotype: F ( results are consistent with previous finding that striatal SNARE complex formation in striatum is not regulated by synucleins (16). We next explored the alternative hypothesis that α-syn might restrict dopamine release by altering the distribution of the vesicular pool in dopamine axons in the dorsal striatum (17).…”

α-Synuclein – Regulator of Exocytosis, Endocytosis, or Both?

“…Studies in mice with knockout of a-, b-and g-synuclein (triple knockout mice) so far show an increase in synaptic transmission only in young (3-months-old) but not in adult mice [43]. However, a more recent study demonstrated an increase in dopamine release in 1 year old triple knockout mice [44], which again argues for a negative regulatory function of a-synuclein in exocytosis ( Table 2).…”