Motor deficits and altered striatal gene expression in aphakia (ak) mice

Mitofusin 2 plays a critical role in maintaining the functional integrity of the neuromuscular-skeletal axis

PURPOSE

Mitofusin 2 (Mfn2) is one of two mitofusins involved in regulating mitochondrial size, shape and function, including mitophagy, an important cellular mechanism to limit oxidative stress. Reduced expression of Mfn2 has been associated with impaired osteoblast differentiation and function and a reduction in the number of viable osteocytes in bone. We hypothesized that the genetic absence of Mfn2 in these cells would increase their susceptibility to aging-associated metabolic stress, leading to a progressive impairment in skeletal homeostasis over time.

METHODS

Mfn2 was selectively deleted in vivo at three different stages of osteoblast lineage commitment by crossing mice in which the Mfn2 gene was floxed with transgenic mice expressing Cre under the control of the promoter for Osterix (OSX), collagen1a1, or DMP1 (Dentin Matrix Acidic Phosphoprotein 1).

RESULTS

Mice in which Mfn2 was deleted using DMP1-cre demonstrated a progressive and dramatic decline in bone mineral density (BMD) beginning at 10 weeks of age (n = 5 for each sex and each genotype from age 10 to 20 weeks). By 15 weeks, there was evidence for a functional decline in muscle performance as assessed using a rotarod apparatus (n = 3; 2 males/ 1 female for each genotype), accompanied by a decline in lean body mass. A marked reduction in trabecular bone mass was evident on bone histomorphometry, and biomechanical testing at 25 weeks (k/o: 2 male/1 female, control 2 male/2 female) revealed severely impaired femur strength. Extensive regional myofiber atrophy and degeneration was observed on skeletal muscle histology. Electron microscopy showed progressive disruption of cellular architecture, with disorganized sarcomeres and a bloated mitochondrial reticulum. There was also evidence of neurodegeneration within the ventral horn and roots of the lumbar spinal cord, which was accompanied by myelin loss and myofiber atrophy. Deletion of Mfn2 using OSX-cre or Col1a1-cre did not result in a musculoskeletal phenotype. Where possible, male and female animals were analyzed separately, but small numbers of animals in each group limited statistical power. For other outcomes, where sex was not considered, small sample sizes might still limit the strength of the observation.

CONCLUSION

Despite known functional overlap of Mfn1 and Mfn2 in some tissues, and their co-expression in bone, muscle and spinal cord, deletion of Mfn2 using the 8 kB DMP1 promoter uncovered an important non-redundant role for Mfn2 in maintaining the neuromuscular/bone axis.

Sensorimotor assessment of the unilateral 6-hydroxydopamine mouse model of Parkinson's disease

Parkinson's disease (PD), the second most common neurodegenerative disorder, is characterized by marked impairments in motor function caused by the progressive loss of dopaminergic neurons in the substantia nigra pars compacta (SNc). Animal models of PD have traditionally been based on toxins, such as 6-hydroxydopamine (6-OHDA) and 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), that selectively lesion dopaminergic neurons. Motor impairments from 6-OHDA lesions of SNc neurons are well characterized in rats, but much less work has been done in mice. In this study, we compare the effectiveness of a series of drug-free behavioral tests in assessing sensorimotor impairments in the unilateral 6-OHDA mouse model, including six tests used for the first time in this PD mouse model (the automated treadmill "DigiGait" test, the challenging beam test, the adhesive removal test, the pole test, the adjusting steps test, and the test of spontaneous activity) and two tests used previously in 6-OHDA-lesioned mice (the limb-use asymmetry "cylinder" test and the manual gait test). We demonstrate that the limb-use asymmetry, challenging beam, pole, adjusting steps, and spontaneous activity tests are all highly robust assays for detecting sensorimotor impairments in the 6-OHDA mouse model. We also discuss the use of the behavioral tests for specific experimental objectives, such as simple screening for well-lesioned mice in studies of PD cellular pathophysiology or comprehensive behavioral analysis in preclinical therapeutic testing using a battery of sensorimotor tests.

Enhanced striatal cholinergic neuronal activity mediates L-DOPA-induced dyskinesia in parkinsonian mice

Treatment of Parkinson disease (PD) with L-3,4-dihydroxyphenylalanine (L-DOPA) dramatically relieves associated motor deficits, but L-DOPA-induced dyskinesias (LID) limit the therapeutic benefit over time. Previous investigations have noted changes in striatal medium spiny neurons, including abnormal activation of extracellular signal-regulated kinase1/2 (ERK). Using two PD models, the traditional 6-hydroxydopamine toxic lesion and a genetic model with nigrostriatal dopaminergic deficits, we found that acute dopamine challenge induces ERK activation in medium spiny neurons in denervated striatum. After repeated L-DOPA treatment, however, ERK activation diminishes in medium spiny neurons and increases in striatal cholinergic interneurons. ERK activation leads to enhanced basal firing rate and stronger excitatory responses to dopamine in striatal cholinergic neurons. Pharmacological blockers of ERK activation inhibit L-DOPA-induced changes in ERK phosphorylation, neuronal excitability, and the behavioral manifestation of LID. In addition, a muscarinic receptor antagonist reduces LID. These data indicate that increased dopamine sensitivity of striatal cholinergic neurons contributes to the expression of LID, which suggests novel therapeutic targets for LID.

Dopamine-dependent motor learning: insight into levodopa's long-duration response

OBJECTIVE

Dopamine (DA) is critical for motor performance, motor learning, and corticostriatal plasticity. The relationship between motor performance and learning, and the role of DA in the mediation of them, however, remain unclear.

METHODS

To examine this question, we took advantage of PITx3-deficient mice (aphakia mice), in which DA in the dorsal striatum is reduced by 90%. PITx3-deficient mice do not display obvious motor deficits in their home cage, but are impaired in motor tasks that require new motor skills. We used the accelerating rotarod as a motor learning task.

RESULTS

We show that the deficiency in motor skill learning in PITx3(-/-) is dramatic and can be rescued with levodopa treatment. In addition, cessation of levodopa treatment after acquisition of the motor skill does not result in an immediate drop in performance. Instead, there is a gradual decline of performance that lasts for a few days, which is not related to levodopa pharmacokinetics. We show that this gradual decline is dependent on the retesting experience.

INTERPRETATION

This observation resembles the long-duration response to levodopa therapy in its slow buildup of improvement after the initiation of therapy and gradual degradation. We hypothesize that motor learning may play a significant, underappreciated role in the symptomatology of Parkinson disease as well as in the therapeutic effects of levodopa. We suggest that the important, yet enigmatic long-duration response to chronic levodopa treatment is a manifestation of rescued motor learning.

Evaluation of the antibradykinetic actions of 5-HT1A agonists using the mouse pole test

To clarify the role and mechanism of the 5-HT1A receptor in modulating extrapyramidal motor disorders, we studied the actions of 5-HT1A agonists in the mouse pole test, a valid model of parkinsonian bradykinesia. Haloperidol markedly delayed pole-descending behavior of mice in the pole test, and this effect was alleviated by the antiparkinsonian agent trihexyphenidyl (a muscarinic antagonist). The selective 5-HT1A agonists, 8-hydroxydipropylaminotetraline (8-OH-DPAT) and tandospirone, significantly attenuated haloperidol-induced bradykinesia in a dose-dependent manner. The alleviation of haloperidol-induced bradykinesia by 8-OH-DPAT was completely antagonized by WAY-100135 (a selective 5-HT1A antagonist), but was unaffected by cerebral 5-HT depletion with p-chlorophenylalanine (PCPA) treatment (300 mg/kg, i.p. for 3 days). These results suggest that 5-HT1A agonists improve extrapyramidal motor disorders associated with antipsychotic treatments by stimulating the postsynaptic 5-HT1A receptor.

Impaired learning and memory in Pitx3 deficient aphakia mice: a genetic model for striatum-dependent cognitive symptoms in Parkinson's disease

Disorders of the basal ganglia such as Parkinson's disease (PD) and Huntington's disease are commonly thought of primarily as motor disorders; however, the cognitive symptoms of these diseases such as executive dysfunction, learning, memory and attention deficits are prominent and often more disabling than the hallmark motor symptoms. Cognitive features of PD are often neglected in preclinical studies of PD, likely due to the lack of available animal models to study them. Aphakia mice, which are deficient in the transcription factor Pitx3, model the selective nigrostriatal DA loss in PD. Here we report that aphakia mice are impaired in striatum-dependent cognitive tasks including rotarod learning, T-maze and inhibitory avoidance tasks, but not the striatum-independent social transmission of food preference task. These results suggest that some neuropsychiatric symptoms in PD are related to the pathophysiology of the disease rather than stress associated with disease burden, or medications used to treat PD. Furthermore aphakia mice may be used as a novel model of non-motor symptoms in PD.