Abstract
Fragile X syndrome (FXS) is the most common form of inherited intellectual disability. It is caused by the overexpansion of cytosine-guanine-guanine (CGG) trinucleotide in Fmr1 gene, resulting in complete loss of the fragile X mental retardation protein (FMRP). Previous studies using Fmr1 knockout (Fmr1 KO) mice have suggested that a N-methyl-D-aspartate receptors (NMDAR) hypofunction in the hippocampal dentate gyrus may partly contribute to cognitive impairments in FXS. Since activation of NMDAR plays an important role in dendritic arborization during neuronal development, we examined whether deficits in NMDAR function are associated with alterations in dendritic complexity in the hippocampal dentate region. The dentate granule cell layer (GCL) presents active postnatal neurogenesis, and consists of a heterogenous neuronal population with gradient ages from the superficial to its deep layer. Here, we show that neurons with multiple primary dendrites that reside in the outer GCL of Fmr1 KO mice display significantly smaller NMDAR excitatory post-synaptic currents (EPSCs) and a higher α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) to NMDA ratio in comparison to their wild-type counterparts. These deficits were associated with a significant decrease in dendritic complexity, with both dendritic length and number of intersections being significantly reduced. In contrast, although neurons with a single primary dendrite resided in the inner GCL of Fmr1 KO mice had a trend toward a reduction in NMDAR EPSCs and a higher AMPA/NMDA ratio, no alterations were found in dendritic complexity at this developmental stage. Our data indicate that the loss of FMRP causes NMDAR deficits and reduced dendritic complexity in granule neurons with multiple primary dendrites which are thought to be more mature in the GCL.
Highlights
Cognitive impairment is a frequently reported issue for patients with fragile X syndrome (FXS), which is the most common form of inherited intellectual disability, and the leading single gene cause of autism spectrum disorder (Rousseau et al, 1991; Kabakus et al, 2006; Alanay et al, 2007)
The current study indicates that a loss of fragile X mental retardation protein (FMRP) results in a significant reduction in N-methyl-D-aspartate receptor (NMDAR)-mediated excitatory post-synaptic currents (EPSCs) and a concomitant decrease in dendritic complexity in hippocampal granule cells
Our findings indicate that these deficits are primarily observed in the mature granule neurons which are with multiple primary dendritic processes arising from the soma, and are primarily located in the outer granule cell layer (GCL)
Summary
Cognitive impairment is a frequently reported issue for patients with fragile X syndrome (FXS), which is the most common form of inherited intellectual disability, and the leading single gene cause of autism spectrum disorder (Rousseau et al, 1991; Kabakus et al, 2006; Alanay et al, 2007). This syndrome is caused by the transcriptional silencing of the fragile X mental retardation 1 (Fmr). The impact of NMDAR on dendritic structure can have functional implications, as neuronal models indicate that dendritic morphology can have a significant impact on neuronal firing patterns (Mainen and Sejnowski, 1996)
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