Constipation, deficit in colon contractions and alpha-synuclein inclusions within the colon precede motor abnormalities and neurodegeneration in the central nervous system in a mouse model of alpha-synucleinopathy

Background: Gastrointestinal dysfunction can affect Parkinson's disease (PD) patients long before the onset of motor symptoms. However, little is known about the relationship between gastrointestinal abnormalities and the development of PD. Contrary to other animal models, the human A53T alpha-synuclein (alpha S) transgenic mice, Line G2-3, develops alpha S-driven neurological and motor impairments after 9 months of age, displaying a long presymptomatic phase free of central nervous system (CNS) dysfunction. Methods: To determine whether this line can be suitable to study constipation as it occurs in prodromal PD, gastrointestinal functionality was assessed in young mice through a multidisciplinary approach, based on behavioral and biochemical analysis combined with electrophysiological recordings of mouse intestinal preparations. Results: We found that the A53T alpha S mice display remarkable signs of gastrointestinal dysfunction that precede motor abnormalities and aS pathology in the CNS by at least 6 months. Young alpha S mice show a drastic delay in food transit along the gastrointestinal tract, of almost 2 h in 3 months old mice that increased to more than 3 h at 6 months. Such impairment was associated with abnormal formation of stools that resulted in less abundant but longer pellets excreted, suggesting a deficit in the intestinal peristalsis. In agreement with this, electrically evoked contractions of the colon, but not of the ileum, showed a reduced motor response in both longitudinal and circular muscle layers in alpha S mice already at 3 months of age, that was mainly due to an impaired cholinergic transmission of the underlying enteric nervous system. Interestingly, the presence of insoluble and aggregated alpha S was found in enteric neurons in both myenteric and submucosal plexi only in the colon of 3 months old alpha S mice, but not in the small intestine, and exacerbated with age, mimicking the increase in transit delay and the contraction deficit showed by behavioral and electrical recordings data. Conclusions: Gastrointestinal dysfunction in A53T alpha S mice represents an early sign of alpha S-driven pathology without concomitant CNS involvement. We believe that this model can be very useful to study disease-modifying strategies that could extend the prodromal phase of PD and halt alpha S pathology from reaching the brain.