Pre-symptomatic detection of chronic motor deficits and genotype prediction in congenic B6.SOD1(G93A) ALS mouse model

Abstract

Amyotrophic lateral sclerosis (ALS) is an incurable progressive paralytic motor neuron disease with limited therapeutic options. Since their creation by Gurney et al. (1994) [Science 264:1772-1775], transgenic superoxide dismutase-1 with glycine to alanine switch at codon 93 (SOD1(G93A)) mice have become the benchmark pre-clinical model for screening ALS therapies. Surprisingly, despite physiological, anatomical, ultrastructural and biochemical evidence of early motor system dysfunction, it has proven difficult to detect motor performance deficits in pre-symptomatic SOD1(G93A) mice. As an alternative to conventional forced motor tests, we investigated the progression of motor performance deficits in freely behaving pre-symptomatic congenic B6.SOD1(G93A) mice. We found that motor performance deficits began several weeks prior to the onset of overt clinical symptoms (postnatal day 45). More importantly, once motor performance deficits manifested, they persisted in parallel with disease progression. In addition, two physical measures of muscle girth revealed progressive hindlimb muscle atrophy that predicted genotype in individual pre-symptomatic mice with 80% accuracy. Together, these data suggest that muscle girth is a reliable and indirect measure of hindlimb muscle denervation and an early, objective marker for disease onset in congenic B6.SOD1(G93A) ALS mice. Moreover, we present regression equations based on hindlimb muscle girth for predicting genotype in future studies using B6.SOD1(G93A) mice. These findings support new objective criteria for clinical disease onset and provide objective measures that require little expertise. These studies demonstrate a cost-effective approach for more thorough evaluation of neuroprotective strategies that seek to disrupt disease mechanisms early in the disease process. To our knowledge, these findings are the first to report early chronic motor performance and physical deficits that are coincident with the earliest known motor dysfunction in any ALS mouse model.

Figure 1. Delayed clinical symptom onset in female B6.SOD1^G93A mice

Kaplan-Meier curves to compare tremor onset and survival time (moribund animals unable to right themselves within 20 s) between male and female B6.SOD1^G93A mice. Clinical symptom onset (tremor in at least 1 hindlimb upon tail suspension) was delayed ~10 days in females (n=39) compared to male mice (n=49). No significant difference in survival time between male and female B6.SOD1^G93A mice was observed.

Figure 2. Progression of motor deficits in B6.SOD1^G93A mice

Line graphs showing time course of disease progression from motor tests. Animals were monitored every 10 d from P45-P125 except for rotarod (P55-P125). For simplicity, all motor performance graphs for wildtype (WT, open circles) and B6.SOD1^G93A (Tg, black squares) were collapsed across sex. (A-D) Open field measures. (E) Paw grip endurance (PaGE) test and (F) Rotarod. The dashed box indicates the time points for post hoc statistical analysis of male and female mice (see Table 2).

Figure 3. Progression of muscle size and body weight deficits in B6.SOD1^G93A mice

Line graphs showing time course of disease progression from physical measures. Animals were monitored every 10 d from P45-P125. For clarity, all muscle girth graphs for each group (wildtype and B6.SOD1^G93A) were collapsed across sex (A-C). The dashed box indicates the time points for post hoc statistical analysis of male and female mice (shaded region in Table 2).

Figure 4. Muscle girth predicts genotype in pre-symptomatic SOD1 ^G93A mice

Line graph showing at each time point the percentage of animals correctly classified as wildtype or transgenic based on calf (d/v) and thigh muscle girth. Notice that 27out of 35 females can be correctly g by P55 whereas 43 out of 54 males can be correctly classified at P65.

Figure 5. Phenotypic diagnostic accuracy in muscle girth but not motor performance measures

Receiver-operated-characteristics (ROC) curves to determine test accuracy were generated for each time point from P45 to P125 for all physical measures (A and B) and motor performance tests (C and D). Notice how rapidly the phenotypic diagnostic accuracy for calf (d/v) muscle girth measures reaches 80% in pre-symptomatic males and females (A and B) and is quickly matched by calf (m/l) and thigh at P65. While motor tasks are not as diagnostically accurate as muscle girth measures (C and D), in general, they are more accurate for males versus female mice. Shaded region indicates high diagnostic accuracy.