MitoPark mice mirror the slow progression of key symptoms and L-DOPA response in Parkinson's disease

Abstract

The MitoPark mouse, in which the mitochondrial transcription factor Tfam is selectively removed in midbrain dopamine (DA) neurons, is a genetic model for Parkinson's disease (PD) that replicates the slow and progressive development of key symptoms. To further validate this model, we have extended both behavioral and biochemical analyses in these animals. We found that vertical movements decline earlier and faster than horizontal movements, possibly modeling the early occurrence of axial, postural instability in PD. L-DOPA induces different locomotor responses depending on the age: in young MitoPark mice the L-DOPA-induced motor activation is small; middle-aged MitoPark mice respond in a dose-dependent manner to L-DOPA, whereas aged MitoPark mice display a double-peaked locomotor response to a high dose of L-DOPA that includes an intermittent period of very low motor activity, similar to the 'on-off' phenomenon in PD. To correlate behavior with biochemical data, we analyzed monoamine levels in three different brain areas that are highly innervated by the DA system: striatum, anterior cortex and olfactory bulb. DA levels declined earlier and faster in striatum than in cortex; only at the latest time-point analyzed, DA levels were found to be significantly lower than control levels in the olfactory bulb. Interestingly, the ratio between homovanillic acid (HVA) and DA differed between regions over time. In striatum and olfactory bulb, the ratio increased steeply indicating increased DA turnover. In contrast, the ratio decreased over time in cortex, revealing important differences between DA cells in substantia nigra and the ventral tegmental area.

Figure 1. Changes in weight, spontaneous vertical activity, horizontal activity and total distance traveled of MitoPark mice as a percentage of control littermates at five different ages

Bars represent mean beam breaks from six to eight animals per group and error bars indicate SEM. Data were analyzed with two-way ANOVA followed by Bonferroni post test and statistically significant differences to aged matched controls are indicated as follows: *, P < 0.05; **, P < 0.01; ***, P < 0.001.

Figure 2. Representative locomotor responses to two different L-DOPA doses in MitoPark and control mice at four different ages: 8 weeks, 16 weeks, 24 weeks and 32 weeks

L-DOPA was injected after 60 min (arrow) of habituation to the activity cages and horizontal activity was monitored for another 4 h.

Figure 3. Mean horizontal and, respective, vertical activities during 60 min following treatment with 4 or 20 mg/kg of L-DOPA of MitoPark mice at four different ages

The drug-induced activity is compared with their spontaneous locomotor activity 60 min before the injection. Bars represent mean beam breaks from six to eight animals per group and error bars indicate SEM. Data were analyzed with two-way ANOVA followed by Bonferroni post test and statistically significant differences to the spontaneous activity are indicated as follows: *, P < 0.05; **, P < 0.01; ***, P < 0.001.

Figure 4. Monoamine levels in striatum of MitoPark mice compared with control littermates at five different ages

Bars represent mean monoamine levels from six to nine animals per group and error bars indicate SEM. Data were tested with two-way ANOVA followed by Bonferroni post test and statistically significant differences to aged matched controls are indicated as follows: *, P < 0.05; **, P < 0.01, ***, P < 0.001.

Figure 5. Monoamine levels in anterior cortex cerebri of MitoPark mice at five different ages compared with their wild-type littermates

Bars represent mean monoamine levels from six to nine animals per group and error bars indicate SEM. Data were tested with two-way ANOVA followed by Bonferroni post test and statistically significant differences to aged matched controls are indicated as follows: *, P < 0.05; **, P < 0.01, ***, P < 0.001.

Figure 6. Monoamine levels in the olfactory bulb of MitoPark mice at five different ages compared with their wild-type littermates

Bars represent mean monoamine levels from six to nine animals per group and error bars indicate SEM. Data were tested with two-way ANOVA followed by Bonferroni post test and statistically significant differences to aged matched controls are indicated as follows: *, P < 0.05.