Role of GSK3β and the A‐Type Potassium Channel in the Behavioral Phenotype of a Mouse Model of Parkinson’s Disease

Abstract

Degeneration of dopaminergic (DA) neurons is a hallmark of Parkinson’s disease (PD). The mechanism involved in the site‐specific dysfunction of DA in the substantia nigra pars compacta (SNpc) but not in the ventral tegmental area (VTA) is not well understood. Glycogen Synthase Kinase 3 beta (GSK3β) activity has been shown to be increased in mouse models of PD and implicated in the human disease. Here we use the rotarod test to evaluate locomotion and balance, the open field test to evaluate anxiety and locomotion and whole cell current studies of DA neurons in brain slices of the midbrain to test the hypothesis that increased GSK3β activity in a mouse model for rapidly progressive PD inhibits A‐Type K+ channel activity specifically in the SNpc compared with the VTA, resulting in motor and non‐motor dysfunction in these mice. Using mice that overexpress A53Tα‐synuclein (A53T) under the pituitary specific PITX3 promoter as an animal model for PD, which developed gait disturbances as early as 7 weeks of age, we determined that A53T mice demonstrated decreased rotarod time compared with controls (134.4±27.0 sec (n=15) vs 287.1±13.0 sec (n=30); p < 0.001). Furthermore, treatment for 3 weeks with a GSK3β inhibitor, TWS‐119, resulted in an increase in rotarod time after 1 week (2.4±0.55 fold (n=5); p = 0.043), which was stable for up to 3 weeks. Furthermore, the open field test demonstrated that A53T mice spent more time in the periphery and traveled less in the center as compared to control mice (78.6±16.3 cm (n=17) vs 177.8±24.0 cm (n=30); p=0.006), and treatment with TWS‐119 led to a decrease in time spent in the periphery and an increase in distance traveled in the center as compared to control, indicating that GSK3β inhibition improved motor and non‐motor functions in this model. Whole‐cell patch‐clamp recordings of DA neurons from brain slices of A53T mice demonstrated a nearly 2‐fold increase in the incidence of spontaneous firing of DA neurons in the SNpc in response to current injection compared to WT (34.8±5.5 pA (n=14) vs 16±5.4 pA (n=6); p=0.04), while firing of DA neurons in slices from TWS‐119 treated A53T mice was not significantly different from WT. There was no difference in firing frequency in DA neurons in the VTA from brain slices from WT and A53T mice. Measurements of A‐Type K+ currents in SNpc of brain slices of A53T mice demonstrated a marked decrease in current density compared to WT (87.4±14.0pA (n=8) vs 164.7±32.4pA (n=19); p<0.001), while A‐Type currents in brain slices from TWS‐119 treated mice were not significantly different from control. Moreover, we identified a putative target site of GSK3β in the Kv4.2 structural subunits of the A‐Type K+ channel. These data support the conclusion that hyperactivity of GSK3β might play a role in the development of a PD behavioral phenotype in the PITX3/A53T mouse model for PD by mediating an increase in excitability of DA neurons via inhibition of the A‐type K channel repolarizing current specifically in DA neurons of the SNpc. This suggests the A‐Type K channel and its subunits as potential therapeutic targets in PD.Support or Funding InformationFunded by The Michael J. Fox Foundation