Elsevier

Brain Research

Volume 971, Issue 1, 2 May 2003, Pages 83-89
Brain Research

Research report
Somatosensory cortical barrel dendritic abnormalities in a mouse model of the fragile X mental retardation syndrome

https://doi.org/10.1016/S0006-8993(03)02363-1Get rights and content

Abstract

The Fragile X mental retardation syndrome is the largest source of inherited mental retardation. The syndrome usually results from the transcriptional silencing of the fragile X mental retardation gene (FMR1). To date the most prominent reported neuronal abnormalities for the fragile X mental retardation syndrome include a higher density of long thin spines similar to those found in sensory deprived and developing tissue, suggesting a possible deficit in pruning of immature spines. Dendrites on spiny stellate cells in the inner 1/3 of the barrel wall in layer IV of the rodent somatosensory cortex have been shown to exhibit developmental pruning similar to that affecting spines. To determine if FMRP plays a role in dendritic development, these neurons were examined in two strains of adult FMRP knockout (FraX) mice. FraX mice in both strains exhibited a greater amount of septa-oriented dendritic material, a morphology consistent with pre-pruning status early in development. This observation suggests that FMRP could be necessary for normal developmentally regulated dendritic pruning.

Introduction

The Fragile X mental retardation syndrome (FXS) is the leading form of inherited mental retardation with an incidence of 1:2000 in males and 1:4000 in females [5]. The syndrome generally results from the transcriptional silencing of FMR1. Its product, the Fragile X Mental Retardation Protein (FMRP) is an alternatively spliced 70–80 kDa protein that contains two KH RNA-binding domains, an RNA-binding RGG box [30], and nuclear localization and export signals [9].

Anatomical studies of FXS humans and fmr1 gene knockout (FraX) mice have suggested a possible role for FMRP in synaptic development. Both FXS humans and FraX mice have been shown to exhibit a higher proportion of long, thin spines consistent with an immature morphology and fewer short, thick spines typical of a mature morphology [7], [16], [18], [19], [20], [27], [28], [40]. Based on the fact that synapses in many brain regions undergo developmental overproduction and subsequent pruning [2], [8], [15], [17], [25], these observations, along with the fact that FMRP has been shown to be upregulated at the synapse in response to class I mGluR stimulation [38], have suggested a possible role for FMRP in synaptic pruning and/or maturation.

Aside from dendritic spine properties, there are very few and inconsistent reports of dendritic abnormalities associated with the absence of FMRP. In vitro studies have reported that cultured hippocampal neurons from FraX mice exhibit fewer primary branches and shorter dendrites [4], [37]. In vivo studies, however, have shown no statistical reduction in dendritic branching of occipital cortical neurons [19]. Although these results conflict, the in vitro studies strongly suggest a possible FMRP role in dendritic development. Furthermore, based on the proposed role of FMRP in synaptic pruning and/or maturation, and since dendrites in some brain regions have been shown to undergo developmental overproduction and subsequent pruning [6], [10], [12], [26], one might speculate that FMRP could play a similar role in dendritic development.

To characterize possible dendritic abnormalities we focused on cells known to undergo developmental dendritic pruning, conceptually paralleling synaptic developmental pruning. Early in the development of layer IV of the mouse somatosensory barrel cortex, spiny stellate neurons in the barrel wall extend dendrites in all directions; then, as they develop, the dendrites projecting towards the barrel center (hollow) flourish while dendrites projecting towards the inter-barrel region (septa) are pruned [12]. This pattern of selective dendritic development makes the barrel cortex an ideal system for examining dendritic abnormalities in regions known to undergo growth/maturation (hollow oriented dendrites) and pruning/removal (septa oriented dendrites) (Fig. 1). The present study examined these spiny stellate neurons in two strains of adult FraX mice to determine whether the absence of FMRP causes dendritic abnormalities in the somatosensory barrel cortex.

Section snippets

Tissue preparation

Twenty-eight sighted FVB (129P-+<Pdeb-rd1>Fmr1<tm1Cgr>) mice (15 FraX, 13 WT) and 18 C57Bl/6 (C57Bl/6-129) mice (9 FraX, 9 WT) (both derived from fmr1 knockout mice obtained from the Dutch–Belgian Fragile X Consortium [1]) were transcardially perfused with 0.9% saline, pH 7.4. After decapitation, each hemisphere of the cerebral cortex was removed, flattened [32], Golgi–Cox impregnated using a standard protocol [11], embedded in celloidin [19] and sectioned at 80 μm. To visualize the barrel

Results

Spiny stellate neurons in FraX and WT mice of both strains did not differ significantly from each other in the amount of dendritic material projecting into the barrel hollow, suggesting that FMRP does not play a critical role in normal dendritic outgrowth. In contrast, FraX mice had significantly more dendritic material projecting towards the septa within the first 25 μm from the soma for FVB (F(1,28)=12.59; P<0.05) mice and within the first 75 μm from the soma for C57Bl/6 (F(1,18)=9.25: P

Discussion

To date the most robust neuronal morphological abnormality reported to be associated with the absence of FMRP is an increase in the number of long thin spines, similar to those reported to be more common after sensory deprivation and in developing tissue. Work from this laboratory and others have demonstrated such spine abnormalities in multiple cerebral cortical regions of both FXS patients and FraX mice [7], [16], [18], [19], [20], [27], [28], [40]. Further research has also found that FMRP

Acknowledgements

The authors thank Kathy Bates for her assistance in manuscript formatting. This work was supported by grants from the FRAXA Research Foundation; NIH/NICHD (HD37175); NIH/NIMH (MH35321); the Society for Neuroscience Minority Neuroscience Fellowship Program–NIH/NIMH (1T32MH20069); and the Illinois–Eastern Iowa District of Kiwanis International Spastic Paralysis and Allied Diseases of the Nervous System Research Foundation.

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