Cognitive deficits in a mouse model of pre-manifest Parkinson's disease

Abstract

Early cognitive deficits are increasingly recognized in patients with Parkinson's disease (PD), and represent an unmet need for the treatment of PD. These early deficits have been difficult to model in mice, and their mechanisms are poorly understood. α-Synuclein is linked to both familial and sporadic forms of PD, and is believed to accumulate in brains of patients with PD before cell loss. Mice expressing human wild-type α-synuclein under the Thy1 promoter (Thy1-aSyn mice) exhibit broad overexpression of α-synuclein throughout the brain and dynamic alterations in dopamine release several months before striatal dopamine loss. We now show that these mice exhibit deficits in cholinergic systems involved in cognition, and cognitive deficits in domains affected in early PD. Together with an increase in extracellular dopamine and a decrease in cortical acetylcholine at 4-6 months of age, Thy1-aSyn mice made fewer spontaneous alternations in the Y-maze and showed deficits in tests of novel object recognition (NOR), object-place recognition, and operant reversal learning, as compared with age-matched wild-type littermates. These data indicate that cognitive impairments that resemble early PD manifestations are reproduced by α-synuclein overexpression in a murine genetic model of PD. With high power to detect drug effects, these anomalies provide a novel platform for testing improved treatments for these pervasive cognitive deficits.

Fig. 1

(A) DI in one-trial object–place recognition in 4–5-month-old mice. (B) Total exploration time of objects in both locations. WT, gray bars; Thy1-aSyn, black bars. ^#P < 0.05 vs. zero (chance level), single-sample t-test. *P < 0.05 vs. WT, unpaired t-test. WT, n = 12; Thy1-aSyn, n = 10.

Fig. 2

NOR performance. (A) Naïve mice were exposed to the two figurines later used in the NOR test. The DI for dog vs. pig is not significantly different between genotypes (t-test, P > 0.05, NS; WT, n = 7; Thy1-aSyn, n = 6). (B) Mice (4–5 months old) previously exposed to two pig figurines were subsequently exposed to pig and dog figurines. DI with dog as a novel object: WT, n = 18; Thy1-aSyn, n = 17. WT, gray bars; Thy1-aSyn, black bars. *P < 0.05 vs. WT, unpaired t-test; ^#P < 0.05, ^###P < 0.001 vs. zero (chance level), single-sample t-test. (C) Percentage of performers [mice exploring the novel object (dog) longer than the familiar object (white portion of bars)] vs. non-performers [mice exploring the familiar object (pig) longer than the novel object (dog) (hatched portion of bars)] in the experiment illustrated in B. **P < 0.01 vs. WT, Fisher's exact test.

Fig. 3

(A) Number of trials to criterion in the acquisition and reversal phases of the operant learning task in 4–5-month-old mice. (B) Time required to respond to the left or right hole when a correct response was made, in both phases of learning. WT, gray bars; Thy1-aSyn, black bars. *P < 0.05 vs. WT at the same phase, repeated-measures anova followed by Fisher's LSD. WT, n = 11; Thy1-aSyn, n = 9.

Fig. 4

Holeboard performance in 3–4-month-old mice. (A–E) Parameters averaged over three trials per day. (A) Reference memory ratio. (B) Latency to complete task. (C) Distance traveled during task. (D) Latency to visit first open hole. (E) Time spent in open holes. (F and G) Parameters measured only for the first trial on each day. (F) Time spent in open holes. (G) Distance traveled during task. (H) Cumulative time needed to retrieve pellets from four open holes. WT, gray circles or bar; Thy1-aSyn, black circles or bar. *P < 0.05, **P < 0.01 vs. WT on same day, Fisher's LSD or t-test. ΔΔP < 0.01 vs. same genotype on day 5, trial 1 (repeated-measure anova followed by Fisher's LSD). WT, n = 9–10; Thy1-aSyn, n = 10–11.

Fig. 5

Double labeling for ChAT and human α-synuclein in the basal nucleus of Meynert. (A–C) ChAT, human α-synuclein and overlay labeling, respectively, in a 5-month-old WT mouse. (D–F) ChAT, human α-synuclein and overlay labeling, respectively, in a 5-month-old Thy1-aSyn mouse. Scale bar: 50 μm. (G) ACh levels in the cortex decrease in Thy1-aSyn mice at 6 months of age. WT, gray bar; Thy1-aSyn, black bar. *P < 0.05 vs. WT, unpaired t-test. WT, n = 9; Thy1-aSyn, n = 7.

Fig. 6

Diagram illustrating the relationship between striatal dopamine (DA) levels and cognitive deficits in the Thy1-aSyn model, on a timeline. The black dots represent time points when measurements were performed. ‘=’ indicates no change in the parameter measured as compared with the wild type; ‘↑’ indicates an increase; and ‘↓’ indicates a decrease.