Comprehensive motor testing in Fmr1-KO mice exposes temporal defects in oromotor coordination

Abstract

Fragile X syndrome (FXS; MIM #300624), a well-recognized form of inherited human mental retardation is caused, in most cases, by a CGG trinucleotide repeat expansion in the 5'-untranslated region of FMR1, resulting in reduced expression of the fragile X mental retardation protein (FMRP). Clinical features include macroorchidism, anxiety, mental retardation, motor coordination, and speech articulation deficits. The Fmr1 knockout (Fmr1-KO) mouse, a mouse model for FXS, has been shown to replicate the macroorchidism, cognitive deficits, and neuroanatomical abnormalities found in human FXS. Here we asked whether Fmr1-KO mice also display appendicular and oromotor deficits comparable to the ataxia and dysarthric speech seen in FXS patients. We employed standard motor tests for balance and appendicular motor coordination, and used a novel long-term fluid-licking assay to investigate oromotor function in Fmr1-KO mice and their wild-type (WT) littermates. Fmr1-KO mice performed equally well as their WT littermates on standard motor tests, with the exception of a raised-beam task. However, Fmr1-KO mice had a significantly slower licking rhythm than their WT littermates. Deficits in rhythmic fluid-licking in Fmr1-KO mice have been linked to cerebellar pathologies. It is believed that balance and motor coordination deficits in FXS patients are caused by cerebellar neurophathologies. The neuronal bases of speech articulation deficits in FXS patients are currently unknown. It is yet to be established whether similar neuronal circuits control rhythmic fluid-licking pattern in mice and speech articulation movement in humans.

Figure 1

The rhythm of fluid-licking movements was significantly slower in Fmr1-KO mice, but long-term aspects of fluid consumption behavior were similar in Fmr1-KO and WT mice. (A) The average ILI duration was significantly increased and hence the rhythm was significantly slower in Fmr1-KO compared to WT littermates. (B) Variability in lick rhythm as determined by the coefficient of variation (CV) ILI is higher in Fmr1-KO mice compared with their WT littermates. Error bars represent standard error of the mean (SE). (C) Log-rank survival plot of mean ILI shows that dynamics of the drinking pattern of FXS mice is different compared with their WT littermates.

Figure 2

Fmr1-KO mice stayed longer on the accelerating rotarod than their WT littermates and showed no deficit in motor learning. Error bars represent standard error of the mean (SE).

Figure 3

Footprint analysis of gait patterns showed no difference between WT and Fmr1-KO mice. (A) Fore-base width, hind-base width, and stride length. (B) Forepaw and hindpaw splay angles. Error bars represent standard error of the mean (SE).

Figure 4

Beam-crossing grip strength and rope-climbing tests reveal only minor motor impairment in Fmr1-KO mice. (A) WT and Fmr1-KO mice crossed a 12-mm-wide square beam in about the same amount of time, but Fmr1-KO mice needed more time to cross an 8-mm-wide square beam than their WT littermates. (B) Fmr1-KO and WT mice had very similar grip strengths. (C) Rope-climbing performance was also quite similar between WT and Fmr1-KO mice. Error bars represent standard error of the mean (SE).