Quantitative measurement of postural sway in mouse models of human neurodegenerative disease

Abstract

Detection of motor dysfunction in genetic mouse models of neurodegenerative disease requires reproducible, standardized and sensitive behavioral assays. We have utilized a center of pressure (CoP) assay in mice to quantify postural sway produced by genetic mutations that affect motor control centers of the brain. As a positive control for postural instability, wild type mice were injected with harmaline, a tremorigenic agent, and the average areas of the 95% confidence ellipse, which measures 95% of the CoP trajectory values recorded in a single trial, were measured. Ellipse area significantly increased in mice treated with increasing doses of harmaline and returned to control values after recovery. We also examined postural sway in mice expressing mutations that mimic frontotemporal dementia with Parkinsonism linked to chromosome 17 (FTDP-17) (T-279, P301L or P301L-nitric oxide synthase 2 (NOS2)(-/-) mice) and that demonstrate motor symptoms. These mice were then compared with a mouse model of Alzheimer's disease (APPSwDI mice) that demonstrates cognitive, but not motor deficits. T-279 and P301L-NOS2(-/-) mice demonstrated a significant increase in CoP ellipse area compared with appropriate wild type control mice or to mice expressing the P301L mutation alone. In contrast, postural instability was significantly reduced in APPSwDI mice that have cognitive deficits but do not have associated motor deficits. The CoP assay provides a simple, sensitive and quantitative tool to detect motor deficits resulting from postural abnormalities in mice and may be useful in understanding the underlying mechanisms of disease.

Figure 1

Basic schematic for the center of pressure apparatus.

Figure 2

Typical graphical representations of 95% ellipse areas. A) Verified resting 95% ellipse area measured for a wild type mouse. Calculated area = 0.069 cm². Verified resting 95% ellipse area measured for a P301L-NOS2^−/− mouse. Calculated area = 1.17 cm². Note :axis scale is the same in A and B. C) Non-usable area measurement from a P301L-NOS2^−/− mouse. Calculated area= 1.54 cm². Axis scale is different from A or B. Note the large deviations outside of the calculated 95% ellipse area which are created by voluntary movements of the mouse.

Figure 3

Dose-response curve for harmaline-induced tremor in wild type mice. Average 95% ellipse areas (± SEM) were calculated for 2 groups of young adult mice (18–30 weeks of age) prior to treatment, after subcutaneous injection of 5 mg/kg (group 1, n= 5 mice) or 10 mg/kg (group 2, n = 5 mice) harmaline and after recovery from treatment. Measurements were completed within 15 min of onset of visual tremors, animals were allowed to recover for 24 hours after tremors had ceased and 95% ellipse areas were again measured to obtain post-recovery values. Statistical significance was determined using the student’s paired t test for each group. # = p < 0.03; * p = 0.001.

Figure 4

Resting postural sway in mouse models of FTDP-17 and AD. Average 95% ellipse areas (±SEM) were calculated for wild type mice (n=5; 66–71 wks), NOS2^−/− mice (n=9; 66wks), P301L-NOS2^−/− mice (n=6; 65–71 wks), P301L mice (n=3; 66 wks) and APPSwDI (n=8; 59 wks). Data is also shown for T-279 mice (n=3; 59 wks) compared to their own littermate control wild type mice (n = 4; 59 wks). * = significant at p < 0.01 compared to wild type; ** = significant at p < 0.005 compared to wild type mice; *** = significant at p < 0.007 compared to littermate control mice. P values were determined using the unpaired student’s t test.

Figure 5

Variability in CoP analysis within strain, over 3 consecutive days of testing and with age of mice. Panel A- The average 95% ellipse area (± SEM) was determined for 5 individual P301L-NOS2^−/− mice (65–71 wks) and 5 individual C57/Bl6 wild type mice (66–72 wks). Panel B- Postural sway measurements for three individual APPSwDI (A–C; 53wks) and 3 individual C57/Bl6 wild type mice (D–E; 10–12 wks) were taken over 3 consecutive days. No significant difference in the average 95% ellipse area was observed over the 3 day period for each individual mouse. Panel C- CoP analysis was performed on groups (4–6 mice/group) of C57/Bl6 wild type mice at different ages. No significant change in the average 95% ellipse area was observed from 10 to 66 weeks of age. A significant decrease in postural sway was observed in very old mice (77–79 wks ) (p < 0.001 by ANOVA).

Figure 6

Analysis of vestibulomotor function using rotorod. Mice used in CoP were also analyzed for generalized motor dysfunction as detected by the rotorod assay. No significant differences were observed between the P301L, P301L-NOS2^−/−, NOS2^−/−, APPSwDI and littermate wild type mice. Large standard errors for P301L and P301L-NOS2^−/− mice reflect the large variability in individual mouse responses.