Deletion of FMR1 in Purkinje Cells Enhances Parallel Fiber LTD, Enlarges Spines, and Attenuates Cerebellar Eyelid Conditioning in Fragile X Syndrome

Abstract

Absence of functional FMRP causes Fragile X syndrome. Abnormalities in synaptic processes in the cerebral cortex and hippocampus contribute to cognitive deficits in Fragile X patients. So far, the potential roles of cerebellar deficits have not been investigated. Here, we demonstrate that both global and Purkinje cell-specific knockouts of Fmr1 show deficits in classical delay eye-blink conditioning in that the percentage of conditioned responses as well as their peak amplitude and peak velocity are reduced. Purkinje cells of these mice show elongated spines and enhanced LTD induction at the parallel fiber synapses that innervate these spines. Moreover, Fragile X patients display the same cerebellar deficits in eye-blink conditioning as the mutant mice. These data indicate that a lack of FMRP leads to cerebellar deficits at both the cellular and behavioral levels and raise the possibility that cerebellar dysfunctions can contribute to motor learning deficits in Fragile X patients.

Rescue of behavioral phenotype and neuronal protrusion morphology in Fmr1 KO mice

“…Prepulse inhibition of startle (PPI) was measured by analysis of eye blink reactions of mice to acoustic stimuli, based on the magnetic distance measurement technique (MDMT) used for eye blink conditioning (Koekkoek et al, 2002;Koekkoek et al, 2005). Adult Fmr1 KO mice (n=8) and wild type littermates (n=9) were anesthetized with an oxygenated mixture of nitrous oxide and isoflurane.…”

“…It is hypothesized that FMRP normally is involved in the inhibition of the translation of several local mRNAs that are important for the mediation of AMPA receptor internalization. Since the amount of AMPA receptors in the postsynaptic density is correlated with protrusion shape, this might also explain the immature protrusion morphology that has been found in different brain areas of both fragile X patients and Fmr1 KO mice (Comery et al, 1997;Galvez and Greenough, 2005;Grossman et al, 2006;Hinton et al, 1991;Koekkoek et al, 2005;Nimchinsky et al, 2001). The mGluR theory has also boosted the search for therapeutic targets for FXS.…”

Early developmental alterations in GABAergic protein expression in fragile X knockout mice

“…Purkinje cells have been shown to possess abnormal dendritic morphology in Fmr1 knockout mice (Koekkoek et al, 2005). It is plausible that GABAergic input from Purkinje cells to the DCN is perturbed, and that this might result in excitotoxic damage to DCN neurons.…”

Reassessment of long-term depression in cerebellar Purkinje cells in mice carrying mutated GluA2 C terminus

“…3A) (19,20). The patch pipette was filled with a Cs + -based internal solution to improve space clamping along PC dendrites by blocking K + channels (15,21). The mean Rm (membrane resistance in MΩ, measured by passing square currents through the patch pipette) in cells recorded with the pipette containing the Cs + -based internal solution was three-to four-times larger than those recorded with a pipette containing a K + -based solution (Table S1).…”

“…This protocol was originally used by Steinberg et al (15), with five PF stimulations at 100 Hz concomitant with somatic depolarization (100-ms duration) applied at 2-s intervals for 1 min. This protocol induced LTD in slices obtained from young mice [postnatal day (P) [20][21][22][23][24][25] in WT, but in neither K882A nor Δ7 PCs (15). In the present study, however, when applied to mature WT mice (3-6 mo), the original protocol of Steinberg et al (15) was ineffective for inducing LTD (98.0 ± 5.7%, n = 4, P = 0.78, paired t test).…”