Expression of Axonal Transport Proteins in Dopaminergic Neurons of the Substantia Nigra in Mouse Models of Preclinical and Clinical Stages of Parkinson’s Disease

Impairment of axonal transport may contribute to the degeneration of dopaminergic (DAergic) neurons in the substantia nigra (SN), a key event in Parkinson’s disease (PD) pathogenesis. Due to the lack of early diagnosis, changes in axonal transport at the preclinical stage can only be studied in PD models. We assessed gene expression (RT-PCR after cell sorting) and protein levels (semiquantitative immunohistochemistry) of axonal transport-related proteins in SN DAergic neurons from mice in subchronic MPTP models of PD (preclinical and clinical stages) and controls. The proteins studied included α-tubulin (Tuba1a), β-tubulin (Tubb3), kinesin (Kif5b, Klc1), dynein (Dynll1, Dync1i1), dynactin (Dctn1), microtubule affinity-regulating kinase 1 (Mark1), and tau (Mapt). In the preclinical stage, Kif5b expression and Kif5B level were increased, possibly to compensatorily preserve anterograde transport. Dynll1 and Tuba1a were upregulated, whereas Dync1i1 and Mapt were downregulated, with no change in tubulin or tau protein levels. In the clinical stage, Klc1, Dync1i1, Dctn1, Mark1, and Mapt expression and Kif5B protein levels decreased. These data indicate that transcriptional alterations in axonal transport proteins precede protein-level changes in DAergic neurons. The upregulation of Kif5B in the preclinical stage suggests that axonal transport proteins may serve as potential early therapeutic targets in PD.