Aldh1a1 and additional markers of dopamine cell heterogeneity in substantia nigra and ventral tegmental area identified as preserved in two transgenic α-synuclein mouse models of neurodegenerative disease

Abstract

Aim: Parkinson’s disease (PD) is characterized by degeneration of midbrain dopamine neurons and synucleinopathy [aggregated alpha-synuclein protein (αSyn)]. The correlation between αSyn pathology and dopamine neuron degeneration remains to be fully established. Mouse models of PD are commonly used to increase knowledge of disease mechanisms. Lately, midbrain dopamine neurons have gained attention as more heterogeneous than previously recognized. With the aim to determine how the midbrain dopamine system in mice is affected in the presence of αSyn pathology, this brain system was studied in two transgenic mouse models of synucleinopathy. Methods: Brain sections from two previously described transgenic mouse lines verified for αSyn pathology through expression of the human αSyn gene (SNCA) under control of the Thy-1 promoter [Thy1-h[A30P]αSyn and Thy1-h[wt]αSyn (L61)], were analyzed using fluorescent in situ hybridization (FISH) and compared with matching sections from wild-type control mice. Probes directed towards mouse and human αSyn mRNA, and a battery of probes towards mRNAs representative of dopamine cell identity and heterogeneity, were implemented. Results: First, validation of αSyn-encoding mRNA was performed. Ample ectopic αSyn mRNA was observed throughout the brain of mice of each transgenic line. Next, midbrain dopamine neurons located in substantia nigra pars compacta (SNc) and ventral tegmental area (VTA) were analyzed using a battery of general and subpopulation-specific dopamine cell markers. This included tyrosine hydroxylase (Th), vesicular monoamine transporter 2 (Vmat2), dopamine transporter (Dat), aldehyde dehydrogenase 1 family member A1 (Aldh1a1), G-protein-activated inward-rectifying potassium channel type 2 (Girk2), calbindin 1 (Calb1), Calb2, gastrin-releasing peptide (Grp), and vesicular glutamate transporter 2 (Vglut2) mRNAs. No difference between transgenic and control mice was observed for any analyzed marker in either the Thy1-h[A30P]αSyn or Thy1-h[wt]αSyn transgenic mouse line. Conclusions: This study demonstrates remarkable robustness of midbrain dopamine cell integrity in the presence of brain-wide ectopic human αSyn in two transgenic mouse models of neurodegenerative disease, motivating further study into mechanisms correlating synucleinopathy with dopamine neuron degeneration in rodent models relevant to PD.