Leukodystrophies are a heterogeneous group of rare, usually genetic, disorders primarily affecting myelin. Named disorders include Pelizaeus-Merzbacher disease, Krabbe disease, and Canavan disease. Mouse models of such rare disorders are important to their understanding and treatment because of the availability of tools and techniques for teasing out the genetic causes and effects in mice. Gait disturbances are common to the leukodystrophies, and quantifying gait in animal models can aid in determining the efficacy of potential therapies.
The ventral plane videography technology of DigiGait enables analysis of gait in laboratory animals, including mouse models of Pelizaeus-Merzbacher disease (PMD). PMD is a hypomyelinating leukodystrophy caused by mutations of the proteolipid protein 1 gene (PLP1), which is located on the X chromosome and encodes the most abundant protein of myelin in the central nervous system. Nearly 60% of PMD results from genomic duplications of a region of the X chromosome that includes the entire PLP1 gene. Researchers recently reported a new murine model of PMD (1) engineered by introducing an X chromosome duplication in the mouse genome that contains Plp1 and neighboring genes commonly duplicated in PMD patients. The Plp1dup mice display altered transcript levels of myelin proteins leading to progressive degeneration of myelin. The study is significant in its demonstration that MICER chromosome engineering techniques can generate a mouse model of human PMD, including altered levels of major myelin proteins and abnormal myelin pathology. The Plp1dup model will be important for insights into how genomic duplication can lead to PMD and for assessing potential treatments (1).
Careful analysis of gait will enable researchers to quantify gait indices and identify a gait phenotype sooner, advance of moribund movement which may have confounded robust kinematic assessment at ~6 months of age [ “Few differences were observed at early time points, but dramatic differences were evident by the 6 month time point.”] The authors reported that the Plp1dup mice moved slowly compared with wild-type littermates (1); certainly their sluggish walking speed would have an effect upon the few metrics of gait reported in Table 1. The authors indicated that some Plp1dup mice required up to 8 seconds to traverse the ~100 cm illuminated walkway [e.g., a walking speed of ~12.2 cm/s], and that the walking speeds varied by up to 60%. This confounder – the wide disparity in walking speeds – must be acknowledged, indeed trumpeted when interpreting differences in gait between subjects, whether mice or man. Video 1 illustrates a mouse traversing the MouseWalk module of DigiGait, which enables the animal to walk voluntarily over a glass walkway, similar to the method described by the authors. Note the hesitance in its walking, particularly given that its healthy cage mate walked twice as fast, voluntarily.
Video 1. Mouse voluntarily traversing the MouseWalk mode of the DigiGait imaging system. Note the natural exploratory behavior of the animal interrupts its walking.
Increased gait variability can be phenotypic of a movement disorder (2), but in the overground paradigm, often even healthy animals walk across an illuminated glass walkway irregularly, with a range of speeds, sometimes with turning, or often not at all. The DigiGait treadmill paradigm, however, keen to consistently detect subtle early gait disturbances (3,4), enables the collection of data over a range of prescribed speeds for numerous strides. Video 2, for example, shows the same mouse from Video 1, but now walking on the DigiGait treadmill at a set speed of 40 cm/s. Data were collected for ~6 seconds, from which over 30 consecutive gait signals could be recorded.
Video 2. Mouse walking on the DigiGait imaging system treadmill. In this instance, the treadmill speed was set to 40 cm/s. A ~6 second bout resulted in over 30 consecutive strides for analysis.
The stride length of a mouse walking ~12 cm/s is significantly shorter than that of the same mouse when walking ~24 cm/s. Many of the metrics of gait in Plp1dup mice reported by the authors are highly dependent on walking speed. Shared limb support, for example, in any subject is highly dependent on walking speed. For frame of reference, you can be sure that the double support time of Jacoby Ellsbury stealing 1st base is significantly shorter than that of David Ortiz as Big Papi walks to first base. [Note – Their shoe size is about the same!]. The data published in Table 1, then, become a descriptive corollary to the indication that “Plp1dup mice moved slowly compared with wild-type littermates.” The increased paw print area reported in the Plp1dup mice (1), however, is interesting.
The authors note that the single duplication leads to increased transcript levels of Plp1 in the brain beginning the second postnatal week. One advantage of the DigiGait imaging system is the flexibility of the instrumentation to assess gait beginning about post-natal day 14. At this age, most healthy mice are enthusiastic treadmill walkers and can walk speeds of at least 16 cm/s. It would be great to populate the set of gait data in Plp1dup mice with quantitative values and companion values for size-matched wild types, even in the developing animals, at known and comparable speeds. We invite researchers who have interesting mouse models to utilize our complimentary service to study gait in their animals. This is a great opportunity to collect pilot data for grant applications. In return for your trust in our commitment to your research goals you receive video, data sets, and commentary on the gait of your animals that can contribute to the advancement of your animal models.
1. Clark et al. Gait abnormalities and progressive myelin degeneration in a new murine model of Pelizaeus-Merzbacher disease with tandem genomic duplication. J Neurosci. 2013 Jul 17;33(29):11788-99.
2. Goldberg et al. Profiling changes in gait dynamics resulting from progressive 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-induced nigrostriatal lesioning. J Neurosci Res. 2011 Oct;89(10):1698-706.
3. Vinsant et al. Characterization of early pathogenesis in the SOD1G93A mouse model of ALS: part I, background and methods. Brain and Behavior 2013; 3(4): 335–350.
4. Vinsant et al. Characterization of early pathogenesis in the SOD1G93A mouse model of ALS: part II, results and discussion. Brain and Behavior 2013; 3(4): 431–457.