Developmental characteristics of dendritic spines in the dentate gyrus of Fmr1 knockout mice

Abstract

Fragile X Syndrome (FXS) is the most common form of inherited mental retardation. The neuroanatomical phenotype of adult FXS patients, as well as adult Fmr1 knockout (KO) mice, includes elevated dendritic spine density and a spine morphology profile in neocortex that resembles younger individuals. Developmental studies in mouse neocortex have revealed a dynamic phenotype that varies with age, especially during the period of synaptic pruning. Here we investigated the hippocampal dentate gyrus to determine if the FXS spine phenotype is similarly tied to periods of maturation and pruning in this brain region. We used high-voltage electron microscopy to characterize Golgi-stained spines along granule cell dendrites in Fmr1 KO and wildtype (WT) mouse dentate gyrus at postnatal days 15, 21, 30, and 60. In contrast to neocortex, dendritic spine density was higher in Fmr1 KO mice across development. Interestingly, neither genotype showed specific phases of synaptogenesis or pruning, potentially explaining the phenotypic differences from neocortex. Similarly, although the KO mice showed a more immature morphological phenotype overall than WT (higher proportion of thin headed spines, lower proportion of mushroom and stubby spines), both genotypes showed gradual development, rather than impairments during specific phases of maturation. Finally, spine length showed a complex developmental pattern that differs from other brain regions examined, suggesting dynamic regulation by FMRP and other brain region-specific proteins. These findings shed new light on FMRP's role in development and highlight the need for new techniques to further understand the mechanisms by which FMRP affects synaptic maturation.

Figure 1

(A) Representative stereoscopic images of dentate granule cell dendrites from 30 day-old wildtype (WT) and Fmr1 knockout (KO) mice. Viewing with red-blue glasses yields a 3-Dimensional image. Scale bar represents 1 µm. (B) Morphology categories used to classify spines. Part B is reprinted from Grossman et al (2006b).

Figure 2

Spine density along dentate granule cell dendrites of WT and Fmr1 KO mice increased with age. Spine density was higher in Fmr1 KO than in WT mice. Error bars represent S.E.M.

Figure 3

The proportion of spines in each morphology category across development in Fmr1 KO and WT mouse dentate gyrus. In both genotypes, the proportion of thin (A/B) spines decreased with age while the proportion of stubby and mushroom-shaped (F/G) spines increased. Across ages, Fmr1 KO mice had a higher proportion of thin single-headed (C/D) spines and a lower proportion of stubby and mushroom-shaped (F/G) spines. Error bars represent S.E.M.

Figure 4

Mean dendritic spine length for predominant spine types along dentate granule cell dendrites in WT and Fmr1 KO mice. Error bars represent S.E.M. (* p < 0.05).