Dopamine-dependent early synaptic and motor dysfunctions induced by a-synuclein in the nigrostriatal circuit

Misfolding and aggregation of alpha-synuclein are specific features of Parkinson's disease and other neurodegenerative diseases defined as synucleinopathies. Parkinson's disease progression has been correlated with the formation and extracellular release of alpha-synuclein aggregates, as well as with their spread from neuron to neuron. Therapeutic interventions in the initial stages of Parkinson's disease require a clear understanding of the mechanisms by which alpha-synuclein disrupts the physiological synaptic and plastic activity of the basal ganglia. For this reason, we identified two early time points to clarify how the intrastriatal injection of alpha-synuclein-preformed fibrils in rodents via retrograde transmission induces time-dependent electrophysiological and behavioural alterations. We found that intrastriatal alpha-synuclein-preformed fibrils perturb the firing rate of dopaminergic neurons in the substantia nigra pars compacta, while the discharge of putative GABAergic cells of the substantia nigra pars reticulata is unchanged. The alpha-synuclein-induced dysregulation of nigrostriatal function also impairs, in a time-dependent manner, the two main forms of striatal synaptic plasticity, long-term potentiation and long-term depression. We also observed an increased glutamatergic transmission measured as an augmented frequency of spontaneous excitatory synaptic currents. These changes in neuronal function in the substantia nigra pars compacta and striatum were observed before overt neuronal death occurred. In an additional set of experiments, we were able to rescue alpha-synuclein-induced alterations of motor function, striatal synaptic plasticity and increased spontaneous excitatory synaptic currents by subchronic treatment with L-DOPA, a precursor of dopamine widely used in the therapy of Parkinson's disease, clearly demonstrating that a dysfunctional dopamine system plays a critical role in the early phases of the disease.Although recent clinical trials targeting amyloid-beta in Alzheimer's disease have shown promising results, there is increasing evidence suggesting that understanding alternative disease pathways that interact with amyloid-beta metabolism and amyloid pathology might be important to halt the clinical deterioration. In particular, there is evidence supporting a critical role of astroglial activation and astrocytosis in Alzheimer's disease. However, so far, no studies have assessed whether astrocytosis is independently related to either amyloid-beta or tau pathology in vivo. To address this question, we determined the levels of the astrocytic marker GFAP in plasma and CSF of 217 amyloid-beta-negative cognitively unimpaired individuals, 71 amyloid-beta-positive cognitively unimpaired individuals, 78 amyloid-beta-positive cognitively impaired individuals, 63 amyloid-beta-negative cognitively impaired individuals and 75 patients with a non-Alzheimer's disease neurodegenerative disorder from the Swedish BioFINDER-2 study. Participants underwent longitudinal amyloid-beta (F-18-flutemetamol) and tau (F-18-RO948) PET as well as cognitive testing. We found that plasma GFAP concentration was significantly increased in all amyloid-beta-positive groups compared with participants without amyloid-beta pathology (P<0.01). In addition, there were significant associations between plasma GFAP with higher amyloid-beta-PET signal in all amyloid-beta-positive groups, but also in cognitively normal individuals with normal amyloid-beta values (P<0.001), which remained significant after controlling for tau-PET signal. Furthermore, plasma GFAP could predict amyloid-beta-PET positivity with an area under the curve of 0.76, which was greater than the performance achieved by CSF GFAP (0.69) and other glial markers (CSF YKL-40: 0.64, soluble TREM2: 0.71). Although correlations were also observed between tau-PET and plasma GFAP, these were no longer significant after controlling for amyloid-beta-PET. In contrast to plasma GFAP, CSF GFAP concentration was significantly increased in non-Alzheimer's disease patients compared to other groups (P50.05) and correlated with amyloid-beta-PET only in amyloid-beta-positive cognitively impaired individuals (P = 0.005). Finally, plasma GFAP was associated with both longitudinal amyloid-beta-PET and cognitive decline, and mediated the effect of amyloid-beta-PET on tau-PET burden, suggesting that astrocytosis secondary to amyloid-beta aggregation might promote tau accumulation. Altogether, these findings indicate that plasma GFAP is an early marker associated with brain amyloid-beta pathology but not tau aggregation, even in cognitively normal individuals with a normal amyloid-beta status. This suggests that plasma GFAP should be incorporated in current hypothetical models of Alzheimer's disease pathogenesis and be used as a non-invasive and accessible tool to detect early astrocytosis secondary to amyloid-beta pathology.

Automated summary

The study identified that alpha-synuclein disrupts dopamine system and striatal synaptic plasticity in early stages of Parkinson's disease. Subchronic treatment with L-DOPA was able to rescue alpha-synuclein-induced alterations, showing the critical role of dysfunctional dopamine system in disease progression.