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Genetic Removal of Matrix Metalloproteinase 9 Rescues the Symptoms of Fragile X Syndrome in a Mouse Model


Fragile X syndrome (FXS), the most common single gene cause of inherited intellectual disability, is caused by a trinucleotide CGG repeat expansion in the 5' untranslated region of the Fragile X Mental Retardation gene (FMR1), which results in promoter hypermethylation and gene silencing. This loss or reduction of the gene product expression, Fragile X Mental Retardation Protein (FMRP), results in the translational dysregulation of specific target mRNAs. Patients with FXS and Fragile X mental retardation gene knockout (ko) mice, an animal model for FXS, exhibit defects in dendritic spine maturation that may underlie cognitive and behavioral abnormalities in FXS, which tend to be at the extreme of the autistic spectrum. Dendritic spines are small protrusions on the surface of dendrites that receive the majority of excitatory synapses in the brain, and changes in their morphology affect synaptic strength. Previous studies have shown that minocycline promotes dendritic spine maturation in primary cultures of hippocampal neurons and in the developing hippocampus of Fmr1 ko mice, which is accompanied by improvements in behavioral performance. Here I confirm the phenotypes previously reported for the Fmr1 ko mouse and investigate a possible link between matrix metalloproteinase-9 (MMP-9) and FMRP by characterizing double Fmr1/Mmp9 ko mice. I show that the deletion of mmp-9 in Fmr1 ko mice rescues the spine development both in vitro and in vivo, and that this deletion also returns mGluR-dependent LTD to the normal levels in Fmr1 ko mice. LTP however remains affected. I also investigated if Fmr1 ko mice exhibit certain behavioral problems, and whether a deletion of mmp-9 can ameliorate these behavioral deficits. The Fmr1 ko mice exhibit increased anxiety and hyperactivity in the open field and defects in social novelty discrimination, most of which are also improved by the removal of mmp-9. Further, I also examined the role of MMP-9 activity in physical traits that are observed in FXS, such as macroorchidism and found that those physical traits are also ameliorated by the genetic removal of mmp-9. Additionally, previous studies have shown the rates of basal protein synthesis to be higher in Fmr1 ko mice. I confirm this and show that the deletion of mmp-9 reduces both the basal levels of protein synthesis and the levels of intracellular signaling as the levels of phosphorylation of Akt, mTOR and eIF4e are reduced to wild type (wt) levels. The levels of MMP-9 and MMP-2 are also upregulated in the hippocampi of the Fmr1 ko mice and since there no differences in the mRNA levels of these two transcripts between Fmr1 ko and wt animals, this upregulation must be due to a translational dysregulation following the loss of FMRP. The fact that most of the FXS phenotypes are reversed by the genetic removal of mmp-9, strongly suggests that increased MMP-9 protein levels underlie these phenotypes and validates the use of specific MMP-9 inhibitors as possible therapeutic avenues for FXS.

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