Progressive models of Parkinson's disease : behavioural and neurochemical analyses

Abstract

Parkinson’s disease (PD) is a common neurodegenerative disorder diagnosed based on the presence of motor symptoms, including resting tremor, postural impairment, bradykinesia and rigidity. The pathological hallmarks are degeneration of midbrain dopaminergic neurons and the accumulation of inclusions containing the misfolded protein α-synuclein (termed “Lewy bodies”) throughout the nervous system. However, when the cardinal motor symptoms of PD emerge already 60-80% of the dopaminergic content has been lost, meaning that patients get diagnosed with PD at a very late disease stage. PD is also associated with many non-motor symptoms, which can appear up to 30 years before the onset of the motor symptoms. In order to fully appreciate the aetiology of PD, including mechanisms underlying the earliest pathological events, valid model systems are needed. Thus, the work of this thesis has focused on the characterization and utilization of progressive models of PD, including the cNurr1 DATCreER knock-out (KO) model (studies I and II), the Lmx1a/bDATCre KO model (study III), and a viral vector-induced model overexpressing α-synuclein in the ventral tegmental area (VTA) (study IV), allowing investigation of the early pathological stages of PD.

We here report that the cNurr1 DATCreER model recapitulated early pathological features of PD, including a progressively reduced expression of several dopaminergic markers in the striatum and ventral midbrain, reduced striatal levels of dopamine and its metabolites, a dopamine fiber pathology not accompanied by midbrain dopaminergic cell death, and profound motor symptoms. cNurr1 DATCreER KO animals also displayed enhanced corticostriatal glutamate release compared with wild-type animals. The PD-related features observed in the cNurr1 DATCreER model were likely caused by an altered expression of mitochondrial genes, in line with mitochondrial dysfunction being a pathological mechanism proposed in human PD. Using this model in combination with the well-established 6-OHDA lesioned mouse model and trace amine-associated receptor 1 (TAAR1) KO mice, a potentially therapeutic role of TAAR1 was revealed in experimental Parkinsonism and L-DOPA-induced side effects.

Ablation of developmental transcription factors Lmx1a and Lmx1b in dopaminergic neurons induced an early progressive striatal pathology resembling key aspects of PD. Lmx1a/b ablation also impaired both motor functions and non-motor functions, including olfactory and short-term memory functions, but left anxiety- or depressive-like behaviours intact. Moreover, Lmx1b was required for the normal function of the autophagic-lysosomal pathway, and, importantly, it was also decreased in midbrain dopaminergic neurons of patients with PD. These findings point toward a clinically relevant role of Lmx1b in human PD, suggesting that its pathological mechanism is related to impaired degradation of defective proteins, which has been proposed as a pathological mechanism in PD.

Further focusing on the non-motor symptoms of PD, α-synuclein was overexpressed in the rat VTA using adeno-associated viral vectors. The VTA contains dopaminergic neurons projecting to the prefrontal cortex and to limbic regions. Our findings revealed a role of the dopaminergic projections of the VTA in emotional long-term memory, which was vulnerable to α-synuclein pathology. This effect was likely mediated via its innervations to limbic regions including the amygdala, hippocampus and nucleus accumbens.

In conclusion, we here show how both pathological and behavioural Parkinson-like features related to early PD can be induced by ablation of transcription factors Nurr1 and Lmx1a/b, and by overexpression of α-synuclein in VTA. Our experiments also highlight the usefulness of the cNurr1 DATCreER KO model as a bilateral model of early PD, and reveal a role of TAAR1 in experimental PD. Hopefully these data will contribute to a better understanding of the early disease stages of PD and ultimately to the development of better treatment regiments.