Enriched environment promotes behavioral and morphological recovery in a mouse model for the fragile X syndrome

Abstract

Fragile X syndrome, the most frequent form of hereditary mental retardation, is due to a mutation of the fragile X mental retardation 1 (FMR1) gene on the X chromosome. Like fragile X patients, FMR1-knockout (FMR1-KO) mice lack the normal fragile X mental retardation protein (FMRP) and show both cognitive alterations and an immature neuronal morphology. We reared FMR1-KO mice in a C57BL/6 background in enriched environmental conditions to examine the possibility that experience-dependent stimulation alleviates their behavioral and neuronal abnormalities. FMR1-KO mice kept in standard cages were hyperactive, displayed an altered pattern of open field exploration, and did not show habituation. Quantitative morphological analyses revealed a reduction in basal dendrite length and branching together with more immature-appearing spines along apical dendrites of layer five pyramidal neurons in the visual cortex. Enrichment largely rescued these behavioral and neuronal abnormalities while increasing alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) glutamate receptor subunit 1 (GluR1) levels in both genotypes. Enrichment did not, however, affect FMRP levels in the WT mice. These data suggest that FMRP-independent pathways activating glutamatergic signaling are preserved in FMR1-KO mice and that they can be elicited by environmental stimulation.

Fig. 1.

Motor activity and object exploration in FMR1-KO (KO) and WT mice reared in standard (S) or enriched (E) conditions. (A) Mean (±SEM) number of sectors crossed during session 1 in the empty open field. KO mice were more active than WT mice irrespective of the rearing conditions experienced during development. (B) Mean (±SEM) peripheral and central sectors crossed during session 1. KO mice reared in standard conditions explored significantly less the central than the peripheral sectors in comparison with the three other mouse groups that similarly explored each type of sector. Enrichment restored the pattern of open-field exploration in KO mice. (C) Mean number of contacts (±SEM) with the five objects during sessions 2–4. KO mice reared in standard conditions did not show habituation of object exploration. Enrichment restored habituation in KO mice. Each session lasted 5 min. *, P < 0. 05.

Fig. 2.

Enrichment rescues basal dendrite structural abnormalities in FMR1-KO mice. (A) Schematic drawing of a typical layer V pyramidal neuron of the occipital cortex, showing the position of basal, apical, and oblique dendrites, and the different branch orders. (B) Basal dendrite branching in FMR1-KO (KO) and WT mice reared in standard (S) or enriched (E) conditions. The mean number of branches (±SEM) was significantly lower in KO mice reared in standard as opposed to enriched conditions. Enrichment significantly increased dendrite branching in KO mice but did not in WT mice. (C–E) Mean dendrite length (±SEM) of basal, oblique, and apical dendrites measured in KO and WT mice reared in standard or enriched conditions. (C) Basal dendrites were significantly shorter in KO mice reared in the standard condition than in the three other groups. Enrichment significantly increased dendrite length in KO mice. Oblique (D) and apical (E) dendrite length did not significantly vary among the four mouse groups. Dendrite length is expressed in micrometers. **, P < 0.01.

Fig. 3.

Spine density (mean ± SEM) along apical dendrites in FMR1-KO (KO) and WT mice reared in standard (S) or enriched (E) conditions. (A) Spine density on apical dendrites was significantly higher in both genotypes reared in the enriched as opposed to the standard condition. (B) Representative Golgi–Cox-stained apical dendrites showing increased spine density in FMR1-KO mice reared in the enriched condition. Spine density along basal and oblique dendrites did not differ significantly according to the genotype or the rearing condition (data not shown). Spine density is expressed as the number of spines per 10 μm of dendrite segments. *, P < 0.05.

Fig. 4.

Enrichment enhances the proportion of mature-appearing spines in FMR1-KO mice. (A) The schema shows a standard categorization of spines along an immature–mature continuum (28) in FMR1-KO (KO) and WT mice reared in standard (S) or enriched (E) conditions. [Reproduced with permission from ref. (Copyright 2000, Oxford University Press).] (B) KO mice reared in the enriched condition exhibited significantly more immature- and fewer immature-appearing spines than KO mice reared in the standard condition. (C) KO and WT mice reared in the enriched condition displayed a similar proportion of immature- and mature-appearing spines. (D) Computer-based reconstruction of the categories of spines most frequently observed in KO and WT mice reared in the standard (Left) and the enriched (Right) condition.

Fig. 5.

GluR1 and FMRP levels in visual cortex. (A Upper) Western blot analysis of GluR1 and β-actin from FMR1-KO (KO) and WT mice reared in standard (S) or enriched (E) conditions. Five animals and two different protein concentrations were analyzed (only one shown), and the intensities of the bands were quantified by using imagequant (Lower). *, P < 0. 05. Enrichment enhanced GluR1 levels in both KO and WT mice, but this effect was more marked in KO mice. Error bars indicate SEM. (B Upper) Western blot analysis of FMRP and β-actin from WT mice reared in standard or enriched conditions. Five animals and two different protein concentrations were analyzed (only one shown), and the intensities of the bands were analyzed by using imagequant (Lower). FMRP level was unaffected by enrichment in WT mice. Error bars indicate SEM.